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| The official guide to swscale for confused developers. | |
| ======================================================== | |
| Current (simplified) Architecture: | |
| --------------------------------- | |
| Input | |
| v | |
| _______OR_________ | |
| / \ | |
| / \ | |
| special converter [Input to YUV converter] | |
| | | | |
| | (8-bit YUV 4:4:4 / 4:2:2 / 4:2:0 / 4:0:0 ) | |
| | | | |
| | v | |
| | Horizontal scaler | |
| | | | |
| | (15-bit 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, ... |