2 #define _MOVIT_YCBCR_H 1
4 // Shared utility functions between YCbCrInput and YCbCr422InterleavedInput.
6 // Conversion from integer to floating-point representation in case of
7 // Y'CbCr is seemingly tricky:
9 // BT.601 page 8 has a table that says that for luma, black is at 16.00_d and
10 // white is at 235.00_d. _d seemingly means “on a floating-point scale from 0
11 // to 255.75”, see §2.4. The .75 is because BT.601 wants to support 10-bit,
12 // but all values are scaled for 8-bit since that's the most common; it is
13 // specified that conversion from 8-bit to 10-bit is done by inserting two
14 // binary zeroes at the end (not repeating bits as one would often do
15 // otherwise). It would seem that BT.601 lives in a world where the idealized
16 // range is really [0,256), not [0,255].
18 // However, GPUs (and by extension Movit) don't work this way. For them,
19 // typically 1.0 maps to the largest possible representable value in the
20 // framebuffer, ie., the range [0.0,1.0] maps to [0,255] for 8-bit
21 // and to [0,1023] (or [0_d,255.75_d] in BT.601 parlance) for 10-bit.
23 // BT.701 (page 5) seems to agree with BT.601; it specifies range 16–235 for
24 // 8-bit luma, and 64–940 for 10-bit luma. This would indicate, for a GPU,
25 // that that for 8-bit mode, the range would be 16/255 to 235/255
26 // (0.06275 to 0.92157), while for 10-bit, it should be 64/1023 to 940/1023
27 // (0.06256 to 0.91887). There's no good compromise here; if you select 8-bit
28 // range, 10-bit goes out of range (white gets to 942), while if you select
29 // 10-bit range, 8-bit gets only to 234, making true white impossible.
31 // We currently support the 8-bit ranges only, since all of our Y'CbCr
32 // handling effects happen to support only 8-bit at the moment. We will need
33 // to fix this eventually, though, with an added field to YCbCrFormat.
35 #include "image_format.h"
42 // Which formula for Y' to use.
43 YCbCrLumaCoefficients luma_coefficients;
45 // If true, assume Y'CbCr coefficients are full-range, ie. go from 0 to 255
46 // instead of the limited 220/225 steps in classic MPEG. For instance,
47 // JPEG uses the Rec. 601 luma coefficients, but full range.
50 // Currently unused, but should be set to 256 for future expansion,
51 // indicating 8-bit interpretation (see file-level comment).
54 // Sampling factors for chroma components. For no subsampling (4:4:4),
56 unsigned chroma_subsampling_x, chroma_subsampling_y;
58 // Positioning of the chroma samples. MPEG-1 and JPEG is (0.5, 0.5);
59 // MPEG-2 and newer typically are (0.0, 0.5).
60 float cb_x_position, cb_y_position;
61 float cr_x_position, cr_y_position;
64 // Convert texel sampling offset for the given chroma channel, given that
65 // chroma position is <pos> (0..1), we are downsampling this chroma channel
66 // by a factor of <subsampling_factor> and the texture we are sampling from
67 // is <resolution> pixels wide/high.
68 float compute_chroma_offset(float pos, unsigned subsampling_factor, unsigned resolution);
70 // Given <ycbcr_format>, compute the values needed to turn Y'CbCr into R'G'B';
71 // first subtract the returned offset, then left-multiply the returned matrix
72 // (the scaling is already folded into it).
73 void compute_ycbcr_matrix(YCbCrFormat ycbcr_format, float *offset, Eigen::Matrix3d *ycbcr_to_rgb);
77 #endif // !defined(_MOVIT_YCBCR_INPUT_H)