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diff --git a/ffmpeg/doc/swscale.txt b/ffmpeg/doc/swscale.txt new file mode 100644 index 0000000..2066009 --- /dev/null +++ b/ffmpeg/doc/swscale.txt @@ -0,0 +1,98 @@ + 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, ... |