usvg/parser/

filter.rs

// Copyright 2022 the Resvg Authors
// SPDX-License-Identifier: Apache-2.0 OR MIT

//! A collection of SVG filters.

use std::collections::HashSet;
use std::str::FromStr;
use std::sync::Arc;

use strict_num::PositiveF32;
use svgtypes::{AspectRatio, Length, LengthUnit as Unit};

use crate::{
    filter::{self, *},
    ApproxZeroUlps, Color, Group, Node, NonEmptyString, NonZeroF32, NonZeroRect, Opacity, Size,
    Units,
};

use super::converter::{self, SvgColorExt};
use super::paint_server::{convert_units, resolve_number};
use super::svgtree::{AId, EId, FromValue, SvgNode};
use super::OptionLog;

impl<'a, 'input: 'a> FromValue<'a, 'input> for filter::ColorInterpolation {
    fn parse(_: SvgNode, _: AId, value: &str) -> Option<Self> {
        match value {
            "sRGB" => Some(filter::ColorInterpolation::SRGB),
            "linearRGB" => Some(filter::ColorInterpolation::LinearRGB),
            _ => None,
        }
    }
}

pub(crate) fn convert(
    node: SvgNode,
    state: &converter::State,
    object_bbox: Option<NonZeroRect>,
    cache: &mut converter::Cache,
) -> Result<Vec<Arc<Filter>>, ()> {
    let value = match node.attribute::<&str>(AId::Filter) {
        Some(v) => v,
        None => return Ok(Vec::new()),
    };

    let mut has_invalid_urls = false;
    let mut filters = Vec::new();

    let create_base_filter_func =
        |kind, filters: &mut Vec<Arc<Filter>>, cache: &mut converter::Cache| {
            // Filter functions, unlike `filter` elements, do not have a filter region.
            // We're currently do not support an unlimited region, so we simply use a fairly large one.
            // This if far from ideal, but good for now.
            // TODO: Should be fixed eventually.
            let mut rect = match kind {
                Kind::DropShadow(_) | Kind::GaussianBlur(_) => {
                    NonZeroRect::from_xywh(-0.5, -0.5, 2.0, 2.0).unwrap()
                }
                _ => NonZeroRect::from_xywh(-0.1, -0.1, 1.2, 1.2).unwrap(),
            };

            let object_bbox = match object_bbox {
                Some(v) => v,
                None => {
                    log::warn!(
                        "Filter '{}' has an invalid region. Skipped.",
                        node.element_id()
                    );
                    return;
                }
            };

            rect = rect.bbox_transform(object_bbox);

            filters.push(Arc::new(Filter {
                id: cache.gen_filter_id(),
                rect,
                primitives: vec![Primitive {
                    rect,
                    // Unlike `filter` elements, filter functions use sRGB colors by default.
                    color_interpolation: ColorInterpolation::SRGB,
                    result: "result".to_string(),
                    kind,
                }],
            }));
        };

    for func in svgtypes::FilterValueListParser::from(value) {
        let func = match func {
            Ok(v) => v,
            Err(e) => {
                // Skip the whole attribute list on error.
                log::warn!("Failed to parse a filter value cause {}. Skipping.", e);
                return Ok(Vec::new());
            }
        };

        match func {
            svgtypes::FilterValue::Blur(std_dev) => create_base_filter_func(
                convert_blur_function(node, std_dev, state),
                &mut filters,
                cache,
            ),
            svgtypes::FilterValue::DropShadow {
                color,
                dx,
                dy,
                std_dev,
            } => create_base_filter_func(
                convert_drop_shadow_function(node, color, dx, dy, std_dev, state),
                &mut filters,
                cache,
            ),
            svgtypes::FilterValue::Brightness(amount) => {
                create_base_filter_func(convert_brightness_function(amount), &mut filters, cache)
            }
            svgtypes::FilterValue::Contrast(amount) => {
                create_base_filter_func(convert_contrast_function(amount), &mut filters, cache)
            }
            svgtypes::FilterValue::Grayscale(amount) => {
                create_base_filter_func(convert_grayscale_function(amount), &mut filters, cache)
            }
            svgtypes::FilterValue::HueRotate(angle) => {
                create_base_filter_func(convert_hue_rotate_function(angle), &mut filters, cache)
            }
            svgtypes::FilterValue::Invert(amount) => {
                create_base_filter_func(convert_invert_function(amount), &mut filters, cache)
            }
            svgtypes::FilterValue::Opacity(amount) => {
                create_base_filter_func(convert_opacity_function(amount), &mut filters, cache)
            }
            svgtypes::FilterValue::Sepia(amount) => {
                create_base_filter_func(convert_sepia_function(amount), &mut filters, cache)
            }
            svgtypes::FilterValue::Saturate(amount) => {
                create_base_filter_func(convert_saturate_function(amount), &mut filters, cache)
            }
            svgtypes::FilterValue::Url(url) => {
                if let Some(link) = node.document().element_by_id(url) {
                    if let Ok(res) = convert_url(link, state, object_bbox, cache) {
                        if let Some(f) = res {
                            filters.push(f);
                        }
                    } else {
                        has_invalid_urls = true;
                    }
                } else {
                    has_invalid_urls = true;
                }
            }
        }
    }

    // If a `filter` attribute had urls pointing to a missing elements
    // and there are no valid filters at all - this is an error.
    //
    // Note that an invalid url is not an error in general.
    if filters.is_empty() && has_invalid_urls {
        return Err(());
    }

    Ok(filters)
}

fn convert_url(
    node: SvgNode,
    state: &converter::State,
    object_bbox: Option<NonZeroRect>,
    cache: &mut converter::Cache,
) -> Result<Option<Arc<Filter>>, ()> {
    let units = convert_units(node, AId::FilterUnits, Units::ObjectBoundingBox);
    let primitive_units = convert_units(node, AId::PrimitiveUnits, Units::UserSpaceOnUse);

    // Check if this element was already converted.
    //
    // Only `userSpaceOnUse` clipPaths can be shared,
    // because `objectBoundingBox` one will be converted into user one
    // and will become node-specific.
    let cacheable = units == Units::UserSpaceOnUse && primitive_units == Units::UserSpaceOnUse;
    if cacheable {
        if let Some(filter) = cache.filters.get(node.element_id()) {
            return Ok(Some(filter.clone()));
        }
    }

    let rect = NonZeroRect::from_xywh(
        resolve_number(
            node,
            AId::X,
            units,
            state,
            Length::new(-10.0, Unit::Percent),
        ),
        resolve_number(
            node,
            AId::Y,
            units,
            state,
            Length::new(-10.0, Unit::Percent),
        ),
        resolve_number(
            node,
            AId::Width,
            units,
            state,
            Length::new(120.0, Unit::Percent),
        ),
        resolve_number(
            node,
            AId::Height,
            units,
            state,
            Length::new(120.0, Unit::Percent),
        ),
    );

    let mut rect = rect
        .log_none(|| {
            log::warn!(
                "Filter '{}' has an invalid region. Skipped.",
                node.element_id()
            )
        })
        .ok_or(())?;

    if units == Units::ObjectBoundingBox {
        if let Some(object_bbox) = object_bbox {
            rect = rect.bbox_transform(object_bbox);
        } else {
            log::warn!("Filters on zero-sized shapes are not allowed.");
            return Err(());
        }
    }

    let node_with_primitives = match find_filter_with_primitives(node) {
        Some(v) => v,
        None => return Err(()),
    };
    let primitives = collect_children(
        &node_with_primitives,
        primitive_units,
        state,
        object_bbox,
        rect,
        cache,
    );
    if primitives.is_empty() {
        return Err(());
    }

    let mut id = NonEmptyString::new(node.element_id().to_string()).ok_or(())?;
    // Generate ID only when we're parsing `objectBoundingBox` filter for the second time.
    if !cacheable && cache.filters.contains_key(id.get()) {
        id = cache.gen_filter_id();
    }
    let id_copy = id.get().to_string();

    let filter = Arc::new(Filter {
        id,
        rect,
        primitives,
    });

    cache.filters.insert(id_copy, filter.clone());

    Ok(Some(filter))
}

fn find_filter_with_primitives<'a>(node: SvgNode<'a, 'a>) -> Option<SvgNode<'a, 'a>> {
    for link in node.href_iter() {
        if link.tag_name() != Some(EId::Filter) {
            log::warn!(
                "Filter '{}' cannot reference '{}' via 'xlink:href'.",
                node.element_id(),
                link.tag_name().unwrap()
            );
            return None;
        }

        if link.has_children() {
            return Some(link);
        }
    }

    None
}

struct FilterResults {
    names: HashSet<String>,
    idx: usize,
}

fn collect_children(
    filter: &SvgNode,
    units: Units,
    state: &converter::State,
    object_bbox: Option<NonZeroRect>,
    filter_region: NonZeroRect,
    cache: &mut converter::Cache,
) -> Vec<Primitive> {
    let mut primitives = Vec::new();

    let mut results = FilterResults {
        names: HashSet::new(),
        idx: 1,
    };

    let scale = if units == Units::ObjectBoundingBox {
        if let Some(object_bbox) = object_bbox {
            object_bbox.size()
        } else {
            // No need to warn. Already checked.
            return Vec::new();
        }
    } else {
        Size::from_wh(1.0, 1.0).unwrap()
    };

    for child in filter.children() {
        let tag_name = match child.tag_name() {
            Some(v) => v,
            None => continue,
        };

        let filter_subregion = match resolve_primitive_region(
            child,
            tag_name,
            units,
            state,
            object_bbox,
            filter_region,
        ) {
            Some(v) => v,
            None => break,
        };

        let kind =
            match tag_name {
                EId::FeDropShadow => convert_drop_shadow(child, scale, &primitives),
                EId::FeGaussianBlur => convert_gaussian_blur(child, scale, &primitives),
                EId::FeOffset => convert_offset(child, scale, &primitives),
                EId::FeBlend => convert_blend(child, &primitives),
                EId::FeFlood => convert_flood(child),
                EId::FeComposite => convert_composite(child, &primitives),
                EId::FeMerge => convert_merge(child, &primitives),
                EId::FeTile => convert_tile(child, &primitives),
                EId::FeImage => convert_image(child, filter_subregion, state, cache),
                EId::FeComponentTransfer => convert_component_transfer(child, &primitives),
                EId::FeColorMatrix => convert_color_matrix(child, &primitives),
                EId::FeConvolveMatrix => convert_convolve_matrix(child, &primitives)
                    .unwrap_or_else(create_dummy_primitive),
                EId::FeMorphology => convert_morphology(child, scale, &primitives),
                EId::FeDisplacementMap => convert_displacement_map(child, scale, &primitives),
                EId::FeTurbulence => convert_turbulence(child),
                EId::FeDiffuseLighting => convert_diffuse_lighting(child, &primitives)
                    .unwrap_or_else(create_dummy_primitive),
                EId::FeSpecularLighting => convert_specular_lighting(child, &primitives)
                    .unwrap_or_else(create_dummy_primitive),
                tag_name => {
                    log::warn!("'{}' is not a valid filter primitive. Skipped.", tag_name);
                    continue;
                }
            };

        let color_interpolation = child
            .find_attribute(AId::ColorInterpolationFilters)
            .unwrap_or_default();

        primitives.push(Primitive {
            rect: filter_subregion,
            color_interpolation,
            result: gen_result(child, &mut results),
            kind,
        });
    }

    // TODO: remove primitives which results are not used

    primitives
}

// TODO: rewrite/simplify/explain/whatever
fn resolve_primitive_region(
    fe: SvgNode,
    kind: EId,
    units: Units,
    state: &converter::State,
    bbox: Option<NonZeroRect>,
    filter_region: NonZeroRect,
) -> Option<NonZeroRect> {
    let x = fe.try_convert_length(AId::X, units, state);
    let y = fe.try_convert_length(AId::Y, units, state);
    let width = fe.try_convert_length(AId::Width, units, state);
    let height = fe.try_convert_length(AId::Height, units, state);

    let region = match kind {
        EId::FeFlood | EId::FeImage => {
            // `feImage` uses the object bbox.
            if units == Units::ObjectBoundingBox {
                let bbox = bbox?;

                // TODO: wrong
                // let ts_bbox = tiny_skia::Rect::new(ts.e, ts.f, ts.a, ts.d).unwrap();

                let r = NonZeroRect::from_xywh(
                    x.unwrap_or(0.0),
                    y.unwrap_or(0.0),
                    width.unwrap_or(1.0),
                    height.unwrap_or(1.0),
                )?;

                return Some(r.bbox_transform(bbox));
            } else {
                filter_region
            }
        }
        _ => filter_region,
    };

    // TODO: Wrong! Does not account rotate and skew.
    if units == Units::ObjectBoundingBox {
        let subregion_bbox = NonZeroRect::from_xywh(
            x.unwrap_or(0.0),
            y.unwrap_or(0.0),
            width.unwrap_or(1.0),
            height.unwrap_or(1.0),
        )?;

        Some(region.bbox_transform(subregion_bbox))
    } else {
        NonZeroRect::from_xywh(
            x.unwrap_or(region.x()),
            y.unwrap_or(region.y()),
            width.unwrap_or(region.width()),
            height.unwrap_or(region.height()),
        )
    }
}

// A malformed filter primitive usually should produce a transparent image.
// But since `FilterKind` structs are designed to always be valid,
// we are using `FeFlood` as fallback.
#[inline(never)]
pub(crate) fn create_dummy_primitive() -> Kind {
    Kind::Flood(Flood {
        color: Color::black(),
        opacity: Opacity::ZERO,
    })
}

#[inline(never)]
fn resolve_input(node: SvgNode, aid: AId, primitives: &[Primitive]) -> Input {
    match node.attribute(aid) {
        Some(s) => {
            let input = parse_in(s);

            // If `in` references an unknown `result` than fallback
            // to previous result or `SourceGraphic`.
            if let Input::Reference(ref name) = input {
                if !primitives.iter().any(|p| p.result == *name) {
                    return if let Some(prev) = primitives.last() {
                        Input::Reference(prev.result.clone())
                    } else {
                        Input::SourceGraphic
                    };
                }
            }

            input
        }
        None => {
            if let Some(prev) = primitives.last() {
                // If `in` is not set and this is not the first primitive
                // than the input is a result of the previous primitive.
                Input::Reference(prev.result.clone())
            } else {
                // If `in` is not set and this is the first primitive
                // than the input is `SourceGraphic`.
                Input::SourceGraphic
            }
        }
    }
}

fn parse_in(s: &str) -> Input {
    match s {
        "SourceGraphic" => Input::SourceGraphic,
        "SourceAlpha" => Input::SourceAlpha,
        "BackgroundImage" | "BackgroundAlpha" | "FillPaint" | "StrokePaint" => {
            log::warn!("{} filter input isn't supported and not planed.", s);
            Input::SourceGraphic
        }
        _ => Input::Reference(s.to_string()),
    }
}

fn gen_result(node: SvgNode, results: &mut FilterResults) -> String {
    match node.attribute::<&str>(AId::Result) {
        Some(s) => {
            // Remember predefined result.
            results.names.insert(s.to_string());
            results.idx += 1;

            s.to_string()
        }
        None => {
            // Generate an unique name for `result`.
            loop {
                let name = format!("result{}", results.idx);
                results.idx += 1;

                if !results.names.contains(&name) {
                    return name;
                }
            }
        }
    }
}

fn convert_blend(fe: SvgNode, primitives: &[Primitive]) -> Kind {
    let mode = fe.attribute(AId::Mode).unwrap_or_default();
    let input1 = resolve_input(fe, AId::In, primitives);
    let input2 = resolve_input(fe, AId::In2, primitives);
    Kind::Blend(Blend {
        mode,
        input1,
        input2,
    })
}

fn convert_color_matrix(fe: SvgNode, primitives: &[Primitive]) -> Kind {
    let kind = convert_color_matrix_kind(fe).unwrap_or_default();
    Kind::ColorMatrix(ColorMatrix {
        input: resolve_input(fe, AId::In, primitives),
        kind,
    })
}

fn convert_color_matrix_kind(fe: SvgNode) -> Option<ColorMatrixKind> {
    match fe.attribute(AId::Type) {
        Some("saturate") => {
            if let Some(list) = fe.attribute::<Vec<f32>>(AId::Values) {
                if !list.is_empty() {
                    let n = crate::f32_bound(0.0, list[0], 1.0);
                    return Some(ColorMatrixKind::Saturate(PositiveF32::new(n).unwrap()));
                } else {
                    return Some(ColorMatrixKind::Saturate(PositiveF32::new(1.0).unwrap()));
                }
            }
        }
        Some("hueRotate") => {
            if let Some(list) = fe.attribute::<Vec<f32>>(AId::Values) {
                if !list.is_empty() {
                    return Some(ColorMatrixKind::HueRotate(list[0]));
                } else {
                    return Some(ColorMatrixKind::HueRotate(0.0));
                }
            }
        }
        Some("luminanceToAlpha") => {
            return Some(ColorMatrixKind::LuminanceToAlpha);
        }
        _ => {
            // Fallback to `matrix`.
            if let Some(list) = fe.attribute::<Vec<f32>>(AId::Values) {
                if list.len() == 20 {
                    return Some(ColorMatrixKind::Matrix(list));
                }
            }
        }
    }

    None
}

fn convert_component_transfer(fe: SvgNode, primitives: &[Primitive]) -> Kind {
    let mut kind = ComponentTransfer {
        input: resolve_input(fe, AId::In, primitives),
        func_r: TransferFunction::Identity,
        func_g: TransferFunction::Identity,
        func_b: TransferFunction::Identity,
        func_a: TransferFunction::Identity,
    };

    for child in fe.children().filter(|n| n.is_element()) {
        if let Some(func) = convert_transfer_function(child) {
            match child.tag_name().unwrap() {
                EId::FeFuncR => kind.func_r = func,
                EId::FeFuncG => kind.func_g = func,
                EId::FeFuncB => kind.func_b = func,
                EId::FeFuncA => kind.func_a = func,
                _ => {}
            }
        }
    }

    Kind::ComponentTransfer(kind)
}

fn convert_transfer_function(node: SvgNode) -> Option<TransferFunction> {
    match node.attribute(AId::Type)? {
        "identity" => Some(TransferFunction::Identity),
        "table" => match node.attribute::<Vec<f32>>(AId::TableValues) {
            Some(values) => Some(TransferFunction::Table(values)),
            None => Some(TransferFunction::Table(Vec::new())),
        },
        "discrete" => match node.attribute::<Vec<f32>>(AId::TableValues) {
            Some(values) => Some(TransferFunction::Discrete(values)),
            None => Some(TransferFunction::Discrete(Vec::new())),
        },
        "linear" => Some(TransferFunction::Linear {
            slope: node.attribute(AId::Slope).unwrap_or(1.0),
            intercept: node.attribute(AId::Intercept).unwrap_or(0.0),
        }),
        "gamma" => Some(TransferFunction::Gamma {
            amplitude: node.attribute(AId::Amplitude).unwrap_or(1.0),
            exponent: node.attribute(AId::Exponent).unwrap_or(1.0),
            offset: node.attribute(AId::Offset).unwrap_or(0.0),
        }),
        _ => None,
    }
}

fn convert_composite(fe: SvgNode, primitives: &[Primitive]) -> Kind {
    let operator = match fe.attribute(AId::Operator).unwrap_or("over") {
        "in" => CompositeOperator::In,
        "out" => CompositeOperator::Out,
        "atop" => CompositeOperator::Atop,
        "xor" => CompositeOperator::Xor,
        "arithmetic" => CompositeOperator::Arithmetic {
            k1: fe.attribute(AId::K1).unwrap_or(0.0),
            k2: fe.attribute(AId::K2).unwrap_or(0.0),
            k3: fe.attribute(AId::K3).unwrap_or(0.0),
            k4: fe.attribute(AId::K4).unwrap_or(0.0),
        },
        _ => CompositeOperator::Over,
    };

    let input1 = resolve_input(fe, AId::In, primitives);
    let input2 = resolve_input(fe, AId::In2, primitives);

    Kind::Composite(Composite {
        operator,
        input1,
        input2,
    })
}

fn convert_convolve_matrix(fe: SvgNode, primitives: &[Primitive]) -> Option<Kind> {
    fn parse_target(target: Option<f32>, order: u32) -> Option<u32> {
        let default_target = (order as f32 / 2.0).floor() as u32;
        let target = target.unwrap_or(default_target as f32) as i32;
        if target < 0 || target >= order as i32 {
            None
        } else {
            Some(target as u32)
        }
    }

    let mut order_x = 3;
    let mut order_y = 3;
    if let Some(value) = fe.attribute::<&str>(AId::Order) {
        let mut s = svgtypes::NumberListParser::from(value);
        let x = s.next().and_then(|a| a.ok()).map(|n| n as i32).unwrap_or(3);
        let y = s.next().and_then(|a| a.ok()).map(|n| n as i32).unwrap_or(x);
        if x > 0 && y > 0 {
            order_x = x as u32;
            order_y = y as u32;
        }
    }

    let mut matrix = Vec::new();
    if let Some(list) = fe.attribute::<Vec<f32>>(AId::KernelMatrix) {
        if list.len() == (order_x * order_y) as usize {
            matrix = list;
        }
    }

    let mut kernel_sum: f32 = matrix.iter().sum();
    // Round up to prevent float precision issues.
    kernel_sum = (kernel_sum * 1_000_000.0).round() / 1_000_000.0;
    if kernel_sum.approx_zero_ulps(4) {
        kernel_sum = 1.0;
    }

    let divisor = fe.attribute(AId::Divisor).unwrap_or(kernel_sum);
    if divisor.approx_zero_ulps(4) {
        return None;
    }

    let bias = fe.attribute(AId::Bias).unwrap_or(0.0);

    let target_x = parse_target(fe.attribute(AId::TargetX), order_x)?;
    let target_y = parse_target(fe.attribute(AId::TargetY), order_y)?;

    let kernel_matrix = ConvolveMatrixData::new(target_x, target_y, order_x, order_y, matrix)?;

    let edge_mode = match fe.attribute(AId::EdgeMode).unwrap_or("duplicate") {
        "none" => EdgeMode::None,
        "wrap" => EdgeMode::Wrap,
        _ => EdgeMode::Duplicate,
    };

    let preserve_alpha = fe.attribute(AId::PreserveAlpha).unwrap_or("false") == "true";

    Some(Kind::ConvolveMatrix(ConvolveMatrix {
        input: resolve_input(fe, AId::In, primitives),
        matrix: kernel_matrix,
        divisor: NonZeroF32::new(divisor).unwrap(),
        bias,
        edge_mode,
        preserve_alpha,
    }))
}

fn convert_displacement_map(fe: SvgNode, scale: Size, primitives: &[Primitive]) -> Kind {
    let parse_channel = |aid| match fe.attribute(aid).unwrap_or("A") {
        "R" => ColorChannel::R,
        "G" => ColorChannel::G,
        "B" => ColorChannel::B,
        _ => ColorChannel::A,
    };

    // TODO: should probably split scale to scale_x and scale_y,
    //       but resvg doesn't support displacement map anyway...
    let scale = (scale.width() + scale.height()) / 2.0;

    Kind::DisplacementMap(DisplacementMap {
        input1: resolve_input(fe, AId::In, primitives),
        input2: resolve_input(fe, AId::In2, primitives),
        scale: fe.attribute(AId::Scale).unwrap_or(0.0) * scale,
        x_channel_selector: parse_channel(AId::XChannelSelector),
        y_channel_selector: parse_channel(AId::YChannelSelector),
    })
}

fn convert_drop_shadow(fe: SvgNode, scale: Size, primitives: &[Primitive]) -> Kind {
    let (std_dev_x, std_dev_y) = convert_std_dev_attr(fe, scale, "2 2");

    let (color, opacity) = fe
        .attribute(AId::FloodColor)
        .unwrap_or_else(svgtypes::Color::black)
        .split_alpha();

    let flood_opacity = fe
        .attribute::<Opacity>(AId::FloodOpacity)
        .unwrap_or(Opacity::ONE);

    Kind::DropShadow(DropShadow {
        input: resolve_input(fe, AId::In, primitives),
        dx: fe.attribute(AId::Dx).unwrap_or(2.0) * scale.width(),
        dy: fe.attribute(AId::Dy).unwrap_or(2.0) * scale.height(),
        std_dev_x,
        std_dev_y,
        color,
        opacity: opacity * flood_opacity,
    })
}

fn convert_flood(fe: SvgNode) -> Kind {
    let (color, opacity) = fe
        .attribute(AId::FloodColor)
        .unwrap_or_else(svgtypes::Color::black)
        .split_alpha();

    let flood_opacity = fe
        .attribute::<Opacity>(AId::FloodOpacity)
        .unwrap_or(Opacity::ONE);

    Kind::Flood(Flood {
        color,
        opacity: opacity * flood_opacity,
    })
}

fn convert_gaussian_blur(fe: SvgNode, scale: Size, primitives: &[Primitive]) -> Kind {
    let (std_dev_x, std_dev_y) = convert_std_dev_attr(fe, scale, "0 0");
    Kind::GaussianBlur(GaussianBlur {
        input: resolve_input(fe, AId::In, primitives),
        std_dev_x,
        std_dev_y,
    })
}

fn convert_std_dev_attr(fe: SvgNode, scale: Size, default: &str) -> (PositiveF32, PositiveF32) {
    let text = fe.attribute(AId::StdDeviation).unwrap_or(default);
    let mut parser = svgtypes::NumberListParser::from(text);

    let n1 = parser.next().and_then(|n| n.ok());
    let n2 = parser.next().and_then(|n| n.ok());
    // `stdDeviation` must have no more than two values.
    // Otherwise we should fallback to `0 0`.
    let n3 = parser.next().and_then(|n| n.ok());

    let (std_dev_x, std_dev_y) = match (n1, n2, n3) {
        (Some(n1), Some(n2), None) => (n1, n2),
        (Some(n1), None, None) => (n1, n1),
        _ => (0.0, 0.0),
    };

    let std_dev_x = (std_dev_x as f32) * scale.width();
    let std_dev_y = (std_dev_y as f32) * scale.height();

    let std_dev_x = PositiveF32::new(std_dev_x).unwrap_or(PositiveF32::ZERO);
    let std_dev_y = PositiveF32::new(std_dev_y).unwrap_or(PositiveF32::ZERO);

    (std_dev_x, std_dev_y)
}

fn convert_image(
    fe: SvgNode,
    filter_subregion: NonZeroRect,
    state: &converter::State,
    cache: &mut converter::Cache,
) -> Kind {
    match convert_image_inner(fe, filter_subregion, state, cache) {
        Some(kind) => kind,
        None => create_dummy_primitive(),
    }
}

fn convert_image_inner(
    fe: SvgNode,
    filter_subregion: NonZeroRect,
    state: &converter::State,
    cache: &mut converter::Cache,
) -> Option<Kind> {
    let rendering_mode = fe
        .find_attribute(AId::ImageRendering)
        .unwrap_or(state.opt.image_rendering);

    if let Some(node) = fe.try_attribute::<SvgNode>(AId::Href) {
        let mut state = state.clone();
        state.fe_image_link = true;
        let mut root = Group::empty();
        super::converter::convert_element(node, &state, cache, &mut root);
        return if root.has_children() {
            root.calculate_bounding_boxes();
            // Transfer node id from group's child to the group itself if needed.
            if let Some(Node::Group(ref mut g)) = root.children.first_mut() {
                if let Some(child2) = g.children.first_mut() {
                    g.id = child2.id().to_string();
                    match child2 {
                        Node::Group(ref mut g2) => g2.id.clear(),
                        Node::Path(ref mut path) => path.id.clear(),
                        Node::Image(ref mut image) => image.id.clear(),
                        Node::Text(ref mut text) => text.id.clear(),
                    }
                }
            }

            Some(Kind::Image(Image { root }))
        } else {
            None
        };
    }

    let href = fe.try_attribute(AId::Href).log_none(|| {
        log::warn!("The 'feImage' element lacks the 'xlink:href' attribute. Skipped.")
    })?;
    let img_data = super::image::get_href_data(href, state)?;
    let actual_size = img_data.actual_size()?;

    let aspect: AspectRatio = fe.attribute(AId::PreserveAspectRatio).unwrap_or_default();

    let mut root = Group::empty();
    super::image::convert_inner(
        img_data,
        cache.gen_image_id().take(),
        true,
        rendering_mode,
        aspect,
        actual_size,
        filter_subregion.translate_to(0.0, 0.0)?,
        cache,
        &mut root,
    );
    root.calculate_bounding_boxes();

    Some(Kind::Image(Image { root }))
}

fn convert_diffuse_lighting(fe: SvgNode, primitives: &[Primitive]) -> Option<Kind> {
    let light_source = convert_light_source(fe)?;
    Some(Kind::DiffuseLighting(DiffuseLighting {
        input: resolve_input(fe, AId::In, primitives),
        surface_scale: fe.attribute(AId::SurfaceScale).unwrap_or(1.0),
        diffuse_constant: fe.attribute(AId::DiffuseConstant).unwrap_or(1.0),
        lighting_color: convert_lighting_color(fe),
        light_source,
    }))
}

fn convert_specular_lighting(fe: SvgNode, primitives: &[Primitive]) -> Option<Kind> {
    let light_source = convert_light_source(fe)?;

    let specular_exponent = fe.attribute(AId::SpecularExponent).unwrap_or(1.0);
    if !(1.0..=128.0).contains(&specular_exponent) {
        // When exponent is out of range, the whole filter primitive should be ignored.
        return None;
    }

    let specular_exponent = crate::f32_bound(1.0, specular_exponent, 128.0);

    Some(Kind::SpecularLighting(SpecularLighting {
        input: resolve_input(fe, AId::In, primitives),
        surface_scale: fe.attribute(AId::SurfaceScale).unwrap_or(1.0),
        specular_constant: fe.attribute(AId::SpecularConstant).unwrap_or(1.0),
        specular_exponent,
        lighting_color: convert_lighting_color(fe),
        light_source,
    }))
}

#[inline(never)]
fn convert_lighting_color(node: SvgNode) -> Color {
    // Color's alpha doesn't affect lighting-color. Simply skip it.
    match node.attribute(AId::LightingColor) {
        Some("currentColor") => {
            node.find_attribute(AId::Color)
                // Yes, a missing `currentColor` resolves to black and not white.
                .unwrap_or(svgtypes::Color::black())
                .split_alpha()
                .0
        }
        Some(value) => {
            if let Ok(c) = svgtypes::Color::from_str(value) {
                c.split_alpha().0
            } else {
                log::warn!("Failed to parse lighting-color value: '{}'.", value);
                Color::white()
            }
        }
        _ => Color::white(),
    }
}

#[inline(never)]
fn convert_light_source(parent: SvgNode) -> Option<LightSource> {
    let child = parent.children().find(|n| {
        matches!(
            n.tag_name(),
            Some(EId::FeDistantLight) | Some(EId::FePointLight) | Some(EId::FeSpotLight)
        )
    })?;

    match child.tag_name() {
        Some(EId::FeDistantLight) => Some(LightSource::DistantLight(DistantLight {
            azimuth: child.attribute(AId::Azimuth).unwrap_or(0.0),
            elevation: child.attribute(AId::Elevation).unwrap_or(0.0),
        })),
        Some(EId::FePointLight) => Some(LightSource::PointLight(PointLight {
            x: child.attribute(AId::X).unwrap_or(0.0),
            y: child.attribute(AId::Y).unwrap_or(0.0),
            z: child.attribute(AId::Z).unwrap_or(0.0),
        })),
        Some(EId::FeSpotLight) => {
            let specular_exponent = child.attribute(AId::SpecularExponent).unwrap_or(1.0);
            let specular_exponent = PositiveF32::new(specular_exponent)
                .unwrap_or_else(|| PositiveF32::new(1.0).unwrap());

            Some(LightSource::SpotLight(SpotLight {
                x: child.attribute(AId::X).unwrap_or(0.0),
                y: child.attribute(AId::Y).unwrap_or(0.0),
                z: child.attribute(AId::Z).unwrap_or(0.0),
                points_at_x: child.attribute(AId::PointsAtX).unwrap_or(0.0),
                points_at_y: child.attribute(AId::PointsAtY).unwrap_or(0.0),
                points_at_z: child.attribute(AId::PointsAtZ).unwrap_or(0.0),
                specular_exponent,
                limiting_cone_angle: child.attribute(AId::LimitingConeAngle),
            }))
        }
        _ => None,
    }
}

fn convert_merge(fe: SvgNode, primitives: &[Primitive]) -> Kind {
    let mut inputs = Vec::new();
    for child in fe.children() {
        inputs.push(resolve_input(child, AId::In, primitives));
    }

    Kind::Merge(Merge { inputs })
}

fn convert_morphology(fe: SvgNode, scale: Size, primitives: &[Primitive]) -> Kind {
    let operator = match fe.attribute(AId::Operator).unwrap_or("erode") {
        "dilate" => MorphologyOperator::Dilate,
        _ => MorphologyOperator::Erode,
    };

    let mut radius_x = PositiveF32::new(scale.width()).unwrap();
    let mut radius_y = PositiveF32::new(scale.height()).unwrap();
    if let Some(list) = fe.attribute::<Vec<f32>>(AId::Radius) {
        let mut rx = 0.0;
        let mut ry = 0.0;
        if list.len() == 2 {
            rx = list[0];
            ry = list[1];
        } else if list.len() == 1 {
            rx = list[0];
            ry = list[0]; // The same as `rx`.
        }

        if rx.approx_zero_ulps(4) && ry.approx_zero_ulps(4) {
            rx = 1.0;
            ry = 1.0;
        }

        // If only one of the values is zero, reset it to 1.0
        // This is not specified in the spec, but this is how Chrome and Safari work.
        if rx.approx_zero_ulps(4) && !ry.approx_zero_ulps(4) {
            rx = 1.0;
        }
        if !rx.approx_zero_ulps(4) && ry.approx_zero_ulps(4) {
            ry = 1.0;
        }

        // Both values must be positive.
        if rx.is_sign_positive() && ry.is_sign_positive() {
            radius_x = PositiveF32::new(rx * scale.width()).unwrap();
            radius_y = PositiveF32::new(ry * scale.height()).unwrap();
        }
    }

    Kind::Morphology(Morphology {
        input: resolve_input(fe, AId::In, primitives),
        operator,
        radius_x,
        radius_y,
    })
}

fn convert_offset(fe: SvgNode, scale: Size, primitives: &[Primitive]) -> Kind {
    Kind::Offset(Offset {
        input: resolve_input(fe, AId::In, primitives),
        dx: fe.attribute(AId::Dx).unwrap_or(0.0) * scale.width(),
        dy: fe.attribute(AId::Dy).unwrap_or(0.0) * scale.height(),
    })
}

fn convert_tile(fe: SvgNode, primitives: &[Primitive]) -> Kind {
    Kind::Tile(Tile {
        input: resolve_input(fe, AId::In, primitives),
    })
}

fn convert_turbulence(fe: SvgNode) -> Kind {
    let mut base_frequency_x = PositiveF32::ZERO;
    let mut base_frequency_y = PositiveF32::ZERO;
    if let Some(list) = fe.attribute::<Vec<f32>>(AId::BaseFrequency) {
        let mut x = 0.0;
        let mut y = 0.0;
        if list.len() == 2 {
            x = list[0];
            y = list[1];
        } else if list.len() == 1 {
            x = list[0];
            y = list[0]; // The same as `x`.
        }

        if x.is_sign_positive() && y.is_sign_positive() {
            base_frequency_x = PositiveF32::new(x).unwrap();
            base_frequency_y = PositiveF32::new(y).unwrap();
        }
    }

    let mut num_octaves = fe.attribute(AId::NumOctaves).unwrap_or(1.0);
    if num_octaves.is_sign_negative() {
        num_octaves = 0.0;
    }

    let kind = match fe.attribute(AId::Type).unwrap_or("turbulence") {
        "fractalNoise" => TurbulenceKind::FractalNoise,
        _ => TurbulenceKind::Turbulence,
    };

    Kind::Turbulence(Turbulence {
        base_frequency_x,
        base_frequency_y,
        num_octaves: num_octaves.round() as u32,
        seed: fe.attribute::<f32>(AId::Seed).unwrap_or(0.0).trunc() as i32,
        stitch_tiles: fe.attribute(AId::StitchTiles) == Some("stitch"),
        kind,
    })
}

#[inline(never)]
fn convert_grayscale_function(amount: f64) -> Kind {
    let amount = amount.min(1.0) as f32;
    Kind::ColorMatrix(ColorMatrix {
        input: Input::SourceGraphic,
        kind: ColorMatrixKind::Matrix(vec![
            (0.2126 + 0.7874 * (1.0 - amount)),
            (0.7152 - 0.7152 * (1.0 - amount)),
            (0.0722 - 0.0722 * (1.0 - amount)),
            0.0,
            0.0,
            (0.2126 - 0.2126 * (1.0 - amount)),
            (0.7152 + 0.2848 * (1.0 - amount)),
            (0.0722 - 0.0722 * (1.0 - amount)),
            0.0,
            0.0,
            (0.2126 - 0.2126 * (1.0 - amount)),
            (0.7152 - 0.7152 * (1.0 - amount)),
            (0.0722 + 0.9278 * (1.0 - amount)),
            0.0,
            0.0,
            0.0,
            0.0,
            0.0,
            1.0,
            0.0,
        ]),
    })
}

#[inline(never)]
fn convert_sepia_function(amount: f64) -> Kind {
    let amount = amount.min(1.0) as f32;
    Kind::ColorMatrix(ColorMatrix {
        input: Input::SourceGraphic,
        kind: ColorMatrixKind::Matrix(vec![
            (0.393 + 0.607 * (1.0 - amount)),
            (0.769 - 0.769 * (1.0 - amount)),
            (0.189 - 0.189 * (1.0 - amount)),
            0.0,
            0.0,
            (0.349 - 0.349 * (1.0 - amount)),
            (0.686 + 0.314 * (1.0 - amount)),
            (0.168 - 0.168 * (1.0 - amount)),
            0.0,
            0.0,
            (0.272 - 0.272 * (1.0 - amount)),
            (0.534 - 0.534 * (1.0 - amount)),
            (0.131 + 0.869 * (1.0 - amount)),
            0.0,
            0.0,
            0.0,
            0.0,
            0.0,
            1.0,
            0.0,
        ]),
    })
}

#[inline(never)]
fn convert_saturate_function(amount: f64) -> Kind {
    let amount = PositiveF32::new(amount as f32).unwrap_or(PositiveF32::ZERO);
    Kind::ColorMatrix(ColorMatrix {
        input: Input::SourceGraphic,
        kind: ColorMatrixKind::Saturate(amount),
    })
}

#[inline(never)]
fn convert_hue_rotate_function(amount: svgtypes::Angle) -> Kind {
    Kind::ColorMatrix(ColorMatrix {
        input: Input::SourceGraphic,
        kind: ColorMatrixKind::HueRotate(amount.to_degrees() as f32),
    })
}

#[inline(never)]
fn convert_invert_function(amount: f64) -> Kind {
    let amount = amount.min(1.0) as f32;
    Kind::ComponentTransfer(ComponentTransfer {
        input: Input::SourceGraphic,
        func_r: TransferFunction::Table(vec![amount, 1.0 - amount]),
        func_g: TransferFunction::Table(vec![amount, 1.0 - amount]),
        func_b: TransferFunction::Table(vec![amount, 1.0 - amount]),
        func_a: TransferFunction::Identity,
    })
}

#[inline(never)]
fn convert_opacity_function(amount: f64) -> Kind {
    let amount = amount.min(1.0) as f32;
    Kind::ComponentTransfer(ComponentTransfer {
        input: Input::SourceGraphic,
        func_r: TransferFunction::Identity,
        func_g: TransferFunction::Identity,
        func_b: TransferFunction::Identity,
        func_a: TransferFunction::Table(vec![0.0, amount]),
    })
}

#[inline(never)]
fn convert_brightness_function(amount: f64) -> Kind {
    let amount = amount as f32;
    Kind::ComponentTransfer(ComponentTransfer {
        input: Input::SourceGraphic,
        func_r: TransferFunction::Linear {
            slope: amount,
            intercept: 0.0,
        },
        func_g: TransferFunction::Linear {
            slope: amount,
            intercept: 0.0,
        },
        func_b: TransferFunction::Linear {
            slope: amount,
            intercept: 0.0,
        },
        func_a: TransferFunction::Identity,
    })
}

#[inline(never)]
fn convert_contrast_function(amount: f64) -> Kind {
    let amount = amount as f32;
    Kind::ComponentTransfer(ComponentTransfer {
        input: Input::SourceGraphic,
        func_r: TransferFunction::Linear {
            slope: amount,
            intercept: -(0.5 * amount) + 0.5,
        },
        func_g: TransferFunction::Linear {
            slope: amount,
            intercept: -(0.5 * amount) + 0.5,
        },
        func_b: TransferFunction::Linear {
            slope: amount,
            intercept: -(0.5 * amount) + 0.5,
        },
        func_a: TransferFunction::Identity,
    })
}

#[inline(never)]
fn convert_blur_function(node: SvgNode, std_dev: Length, state: &converter::State) -> Kind {
    let std_dev = PositiveF32::new(super::units::convert_user_length(
        std_dev,
        node,
        AId::Dx,
        state,
    ))
    .unwrap_or(PositiveF32::ZERO);
    Kind::GaussianBlur(GaussianBlur {
        input: Input::SourceGraphic,
        std_dev_x: std_dev,
        std_dev_y: std_dev,
    })
}

#[inline(never)]
fn convert_drop_shadow_function(
    node: SvgNode,
    color: Option<svgtypes::Color>,
    dx: Length,
    dy: Length,
    std_dev: Length,
    state: &converter::State,
) -> Kind {
    let std_dev = PositiveF32::new(super::units::convert_user_length(
        std_dev,
        node,
        AId::Dx,
        state,
    ))
    .unwrap_or(PositiveF32::ZERO);

    let (color, opacity) = color
        .unwrap_or_else(|| {
            node.find_attribute(AId::Color)
                .unwrap_or_else(svgtypes::Color::black)
        })
        .split_alpha();

    Kind::DropShadow(DropShadow {
        input: Input::SourceGraphic,
        dx: super::units::convert_user_length(dx, node, AId::Dx, state),
        dy: super::units::convert_user_length(dy, node, AId::Dy, state),
        std_dev_x: std_dev,
        std_dev_y: std_dev,
        color,
        opacity,
    })
}