mirror of https://github.com/FFmpeg/FFmpeg.git
You can not select more than 25 topics
Topics must start with a letter or number, can include dashes ('-') and can be up to 35 characters long.
98 lines
4.6 KiB
98 lines
4.6 KiB
The official guide to swscale for confused developers. |
|
======================================================== |
|
|
|
Current (simplified) Architecture: |
|
--------------------------------- |
|
Input |
|
v |
|
_______OR_________ |
|
/ \ |
|
/ \ |
|
special converter [Input to YUV converter] |
|
| | |
|
| (8bit YUV 4:4:4 / 4:2:2 / 4:2:0 / 4:0:0 ) |
|
| | |
|
| v |
|
| Horizontal scaler |
|
| | |
|
| (15bit YUV 4:4:4 / 4:2:2 / 4:2:0 / 4:1:1 / 4:0:0 ) |
|
| | |
|
| v |
|
| Vertical scaler and output converter |
|
| | |
|
v v |
|
output |
|
|
|
|
|
Swscale has 2 scaler paths. Each side must be capable of handling |
|
slices, that is, consecutive non-overlapping rectangles of dimension |
|
(0,slice_top) - (picture_width, slice_bottom). |
|
|
|
special converter |
|
These generally are unscaled converters of common |
|
formats, like YUV 4:2:0/4:2:2 -> RGB12/15/16/24/32. Though it could also |
|
in principle contain scalers optimized for specific common cases. |
|
|
|
Main path |
|
The main path is used when no special converter can be used. The code |
|
is designed as a destination line pull architecture. That is, for each |
|
output line the vertical scaler pulls lines from a ring buffer. When |
|
the ring buffer does not contain the wanted line, then it is pulled from |
|
the input slice through the input converter and horizontal scaler. |
|
The result is also stored in the ring buffer to serve future vertical |
|
scaler requests. |
|
When no more output can be generated because lines from a future slice |
|
would be needed, then all remaining lines in the current slice are |
|
converted, horizontally scaled and put in the ring buffer. |
|
[This is done for luma and chroma, each with possibly different numbers |
|
of lines per picture.] |
|
|
|
Input to YUV Converter |
|
When the input to the main path is not planar 8 bits per component YUV or |
|
8-bit gray, it is converted to planar 8-bit YUV. Two sets of converters |
|
exist for this currently: One performs horizontal downscaling by 2 |
|
before the conversion, the other leaves the full chroma resolution, |
|
but is slightly slower. The scaler will try to preserve full chroma |
|
when the output uses it. It is possible to force full chroma with |
|
SWS_FULL_CHR_H_INP even for cases where the scaler thinks it is useless. |
|
|
|
Horizontal scaler |
|
There are several horizontal scalers. A special case worth mentioning is |
|
the fast bilinear scaler that is made of runtime-generated MMXEXT code |
|
using specially tuned pshufw instructions. |
|
The remaining scalers are specially-tuned for various filter lengths. |
|
They scale 8-bit unsigned planar data to 16-bit signed planar data. |
|
Future >8 bits per component inputs will need to add a new horizontal |
|
scaler that preserves the input precision. |
|
|
|
Vertical scaler and output converter |
|
There is a large number of combined vertical scalers + output converters. |
|
Some are: |
|
* unscaled output converters |
|
* unscaled output converters that average 2 chroma lines |
|
* bilinear converters (C, MMX and accurate MMX) |
|
* arbitrary filter length converters (C, MMX and accurate MMX) |
|
And |
|
* Plain C 8-bit 4:2:2 YUV -> RGB converters using LUTs |
|
* Plain C 17-bit 4:4:4 YUV -> RGB converters using multiplies |
|
* MMX 11-bit 4:2:2 YUV -> RGB converters |
|
* Plain C 16-bit Y -> 16-bit gray |
|
... |
|
|
|
RGB with less than 8 bits per component uses dither to improve the |
|
subjective quality and low-frequency accuracy. |
|
|
|
|
|
Filter coefficients: |
|
-------------------- |
|
There are several different scalers (bilinear, bicubic, lanczos, area, |
|
sinc, ...). Their coefficients are calculated in initFilter(). |
|
Horizontal filter coefficients have a 1.0 point at 1 << 14, vertical ones at |
|
1 << 12. The 1.0 points have been chosen to maximize precision while leaving |
|
a little headroom for convolutional filters like sharpening filters and |
|
minimizing SIMD instructions needed to apply them. |
|
It would be trivial to use a different 1.0 point if some specific scaler |
|
would benefit from it. |
|
Also, as already hinted at, initFilter() accepts an optional convolutional |
|
filter as input that can be used for contrast, saturation, blur, sharpening |
|
shift, chroma vs. luma shift, ...
|
|
|