| 1 | The official guide to swscale for confused developers. |
| 2 | ======================================================== |
| 3 | |
| 4 | Current (simplified) Architecture: |
| 5 | --------------------------------- |
| 6 | Input |
| 7 | v |
| 8 | _______OR_________ |
| 9 | / \ |
| 10 | / \ |
| 11 | special converter [Input to YUV converter] |
| 12 | | | |
| 13 | | (8bit YUV 4:4:4 / 4:2:2 / 4:2:0 / 4:0:0 ) |
| 14 | | | |
| 15 | | v |
| 16 | | Horizontal scaler |
| 17 | | | |
| 18 | | (15bit YUV 4:4:4 / 4:2:2 / 4:2:0 / 4:1:1 / 4:0:0 ) |
| 19 | | | |
| 20 | | v |
| 21 | | Vertical scaler and output converter |
| 22 | | | |
| 23 | v v |
| 24 | output |
| 25 | |
| 26 | |
| 27 | Swscale has 2 scaler paths. Each side must be capable of handling |
| 28 | slices, that is, consecutive non-overlapping rectangles of dimension |
| 29 | (0,slice_top) - (picture_width, slice_bottom). |
| 30 | |
| 31 | special converter |
| 32 | These generally are unscaled converters of common |
| 33 | formats, like YUV 4:2:0/4:2:2 -> RGB12/15/16/24/32. Though it could also |
| 34 | in principle contain scalers optimized for specific common cases. |
| 35 | |
| 36 | Main path |
| 37 | The main path is used when no special converter can be used. The code |
| 38 | is designed as a destination line pull architecture. That is, for each |
| 39 | output line the vertical scaler pulls lines from a ring buffer. When |
| 40 | the ring buffer does not contain the wanted line, then it is pulled from |
| 41 | the input slice through the input converter and horizontal scaler. |
| 42 | The result is also stored in the ring buffer to serve future vertical |
| 43 | scaler requests. |
| 44 | When no more output can be generated because lines from a future slice |
| 45 | would be needed, then all remaining lines in the current slice are |
| 46 | converted, horizontally scaled and put in the ring buffer. |
| 47 | [This is done for luma and chroma, each with possibly different numbers |
| 48 | of lines per picture.] |
| 49 | |
| 50 | Input to YUV Converter |
| 51 | When the input to the main path is not planar 8 bits per component YUV or |
| 52 | 8-bit gray, it is converted to planar 8-bit YUV. Two sets of converters |
| 53 | exist for this currently: One performs horizontal downscaling by 2 |
| 54 | before the conversion, the other leaves the full chroma resolution, |
| 55 | but is slightly slower. The scaler will try to preserve full chroma |
| 56 | when the output uses it. It is possible to force full chroma with |
| 57 | SWS_FULL_CHR_H_INP even for cases where the scaler thinks it is useless. |
| 58 | |
| 59 | Horizontal scaler |
| 60 | There are several horizontal scalers. A special case worth mentioning is |
| 61 | the fast bilinear scaler that is made of runtime-generated MMXEXT code |
| 62 | using specially tuned pshufw instructions. |
| 63 | The remaining scalers are specially-tuned for various filter lengths. |
| 64 | They scale 8-bit unsigned planar data to 16-bit signed planar data. |
| 65 | Future >8 bits per component inputs will need to add a new horizontal |
| 66 | scaler that preserves the input precision. |
| 67 | |
| 68 | Vertical scaler and output converter |
| 69 | There is a large number of combined vertical scalers + output converters. |
| 70 | Some are: |
| 71 | * unscaled output converters |
| 72 | * unscaled output converters that average 2 chroma lines |
| 73 | * bilinear converters (C, MMX and accurate MMX) |
| 74 | * arbitrary filter length converters (C, MMX and accurate MMX) |
| 75 | And |
| 76 | * Plain C 8-bit 4:2:2 YUV -> RGB converters using LUTs |
| 77 | * Plain C 17-bit 4:4:4 YUV -> RGB converters using multiplies |
| 78 | * MMX 11-bit 4:2:2 YUV -> RGB converters |
| 79 | * Plain C 16-bit Y -> 16-bit gray |
| 80 | ... |
| 81 | |
| 82 | RGB with less than 8 bits per component uses dither to improve the |
| 83 | subjective quality and low-frequency accuracy. |
| 84 | |
| 85 | |
| 86 | Filter coefficients: |
| 87 | -------------------- |
| 88 | There are several different scalers (bilinear, bicubic, lanczos, area, |
| 89 | sinc, ...). Their coefficients are calculated in initFilter(). |
| 90 | Horizontal filter coefficients have a 1.0 point at 1 << 14, vertical ones at |
| 91 | 1 << 12. The 1.0 points have been chosen to maximize precision while leaving |
| 92 | a little headroom for convolutional filters like sharpening filters and |
| 93 | minimizing SIMD instructions needed to apply them. |
| 94 | It would be trivial to use a different 1.0 point if some specific scaler |
| 95 | would benefit from it. |
| 96 | Also, as already hinted at, initFilter() accepts an optional convolutional |
| 97 | filter as input that can be used for contrast, saturation, blur, sharpening |
| 98 | shift, chroma vs. luma shift, ... |