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99 lines
4.6 KiB
99 lines
4.6 KiB
The official guide to swscale for confused developers. |
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Current (simplified) Architecture: |
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--------------------------------- |
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Input |
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v |
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_______OR_________ |
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/ \ |
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/ \ |
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special converter [Input to YUV converter] |
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| | |
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| (8bit YUV 4:4:4 / 4:2:2 / 4:2:0 / 4:0:0 ) |
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| v |
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| Horizontal scaler |
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| (15bit YUV 4:4:4 / 4:2:2 / 4:2:0 / 4:1:1 / 4:0:0 ) |
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| v |
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| Vertical scaler and output converter |
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v v |
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output |
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Swscale has 2 scaler paths. Each side must be capable of handling |
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slices, that is, consecutive non-overlapping rectangles of dimension |
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(0,slice_top) - (picture_width, slice_bottom). |
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special converter |
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These generally are unscaled converters of common |
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formats, like YUV 4:2:0/4:2:2 -> RGB15/16/24/32. Though it could also |
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in principle contain scalers optimized for specific common cases. |
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Main path |
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The main path is used when no special converter can be used. The code |
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is designed as a destination line pull architecture. That is, for each |
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output line the vertical scaler pulls lines from a ring buffer. When |
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the ring buffer does not contain the wanted line, then it is pulled from |
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the input slice through the input converter and horizontal scaler. |
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The result is also stored in the ring buffer to serve future vertical |
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scaler requests. |
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When no more output can be generated because lines from a future slice |
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would be needed, then all remaining lines in the current slice are |
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converted, horizontally scaled and put in the ring buffer. |
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[This is done for luma and chroma, each with possibly different numbers |
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of lines per picture.] |
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Input to YUV Converter |
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When the input to the main path is not planar 8 bits per component YUV or |
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8-bit gray, it is converted to planar 8-bit YUV. Two sets of converters |
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exist for this currently: One performs horizontal downscaling by 2 |
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before the conversion, the other leaves the full chroma resolution, |
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but is slightly slower. The scaler will try to preserve full chroma |
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when the output uses it. It is possible to force full chroma with |
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SWS_FULL_CHR_H_INP even for cases where the scaler thinks it is useless. |
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Horizontal scaler |
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There are several horizontal scalers. A special case worth mentioning is |
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the fast bilinear scaler that is made of runtime-generated MMX2 code |
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using specially tuned pshufw instructions. |
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The remaining scalers are specially-tuned for various filter lengths. |
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They scale 8-bit unsigned planar data to 16-bit signed planar data. |
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Future >8 bits per component inputs will need to add a new horizontal |
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scaler that preserves the input precision. |
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Vertical scaler and output converter |
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There is a large number of combined vertical scalers + output converters. |
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Some are: |
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* unscaled output converters |
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* unscaled output converters that average 2 chroma lines |
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* bilinear converters (C, MMX and accurate MMX) |
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* arbitrary filter length converters (C, MMX and accurate MMX) |
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And |
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* Plain C 8-bit 4:2:2 YUV -> RGB converters using LUTs |
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* Plain C 17-bit 4:4:4 YUV -> RGB converters using multiplies |
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* MMX 11-bit 4:2:2 YUV -> RGB converters |
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* Plain C 16-bit Y -> 16-bit gray |
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... |
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RGB with less than 8 bits per component uses dither to improve the |
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subjective quality and low-frequency accuracy. |
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Filter coefficients: |
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-------------------- |
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There are several different scalers (bilinear, bicubic, lanczos, area, |
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sinc, ...). Their coefficients are calculated in initFilter(). |
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Horizontal filter coefficients have a 1.0 point at 1 << 14, vertical ones at |
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1 << 12. The 1.0 points have been chosen to maximize precision while leaving |
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a little headroom for convolutional filters like sharpening filters and |
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minimizing SIMD instructions needed to apply them. |
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It would be trivial to use a different 1.0 point if some specific scaler |
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would benefit from it. |
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Also, as already hinted at, initFilter() accepts an optional convolutional |
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filter as input that can be used for contrast, saturation, blur, sharpening |
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shift, chroma vs. luma shift, ... |
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