Module upsampler

Up-sampling routines

The main upsampling method is a bi-linear interpolation or a “triangle filter “ or libjpeg turbo fancy_upsampling which is a good compromise between speed and visual quality

The filter

Each output pixel is made from (3*A+B)/4 where A is the original pixel closer to the output and B is the one further.

+---+---+
| A | B |
+---+---+
+-+-+-+-+
| |P| | |
+-+-+-+-+

Horizontal Bi-linear filter

|---+-----------+---+
|   |           |   |
| A | |p1 | p2| | B |
|   |           |   |
|---+-----------+---+

For a horizontal bi-linear it’s trivial to implement,

A becomes the input closest to the output.

B varies depending on output.

• For odd positions, input is the next pixel after A
• For even positions, input is the previous value before A.

We iterate in a classic 1-D sliding window with a window of 3. For our sliding window approach, A is the 1st and B is either the 0th term or 2nd term depending on position we are writing.(see scalar code).

For vector code see module sse for explanation.

Vertical bi-linear.

Vertical up-sampling is a bit trickier.

+----+----+
| A1 | A2 |
+----+----+
+----+----+
| p1 | p2 |
+----+-+--+
+----+-+--+
| p3 | p4 |
+----+-+--+
+----+----+
| B1 | B2 |
+----+----+

For p1

• A1 is given a weight of 3 and B1 is given a weight of 1.

For p3

• B1 is given a weight of 3 and A1 is given a weight of 1

Horizontal vertical downsampling/chroma quartering.

Carry out a vertical filter in the first pass, then a horizontal filter in the second pass.

Modules

scalar 🔒

Functions

choose_horizontal_samp_function

choose_hv_samp_function

choose_v_samp_function

generic_sampler

upsample_no_op

Upsample nothing