Commit | Line | Data |
---|---|---|
2ba45a60 DM |
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, ... |