Prepare for better understanding of 10- and 12-bit Y'CbCr.

[movit] / ycbcr.h

diff --git a/ycbcr.h b/ycbcr.h
index 7e5891f741907b48c5b00c52fb733d0614d4b8ef..9179b1958a5cdcc73e1a5b74132a9b2bab45d3c8 100644 (file)
--- a/ycbcr.h
+++ b/ycbcr.h
@@ -2,6 +2,35 @@
 #define _MOVIT_YCBCR_H 1
 
 // Shared utility functions between YCbCrInput and YCbCr422InterleavedInput.
+//
+// Conversion from integer to floating-point representation in case of
+// Y'CbCr is seemingly tricky:
+//
+// BT.601 page 8 has a table that says that for luma, black is at 16.00_d and
+// white is at 235.00_d. _d seemingly means “on a floating-point scale from 0
+// to 255.75”, see §2.4. The .75 is because BT.601 wants to support 10-bit,
+// but all values are scaled for 8-bit since that's the most common; it is
+// specified that conversion from 8-bit to 10-bit is done by inserting two
+// binary zeroes at the end (not repeating bits as one would often do
+// otherwise). It would seem that BT.601 lives in a world where the idealized
+// range is really [0,256), not [0,255].
+//
+// However, GPUs (and by extension Movit) don't work this way. For them,
+// typically 1.0 maps to the largest possible representable value in the
+// framebuffer, ie., the range [0.0,1.0] maps to [0,255] for 8-bit
+// and to [0,1023] (or [0_d,255.75_d] in BT.601 parlance) for 10-bit.
+//
+// BT.701 (page 5) seems to agree with BT.601; it specifies range 16–235 for
+// 8-bit luma, and 64–940 for 10-bit luma. This would indicate, for a GPU,
+// that that for 8-bit mode, the range would be 16/255 to 235/255
+// (0.06275 to 0.92157), while for 10-bit, it should be 64/1023 to 940/1023
+// (0.06256 to 0.91887). There's no good compromise here; if you select 8-bit
+// range, 10-bit goes out of range (white gets to 942), while if you select
+// 10-bit range, 8-bit gets only to 234, making true white impossible.
+//
+// We currently support the 8-bit ranges only, since all of our Y'CbCr
+// handling effects happen to support only 8-bit at the moment. We will need
+// to fix this eventually, though, with an added field to YCbCrFormat.
 
 #include "image_format.h"
 
@@ -18,6 +47,10 @@ struct YCbCrFormat {
        // JPEG uses the Rec. 601 luma coefficients, but full range.
        bool full_range;
 
+       // Currently unused, but should be set to 256 for future expansion,
+       // indicating 8-bit interpretation (see file-level comment).
+       int num_levels;
+
        // Sampling factors for chroma components. For no subsampling (4:4:4),
        // set both to 1.
        unsigned chroma_subsampling_x, chroma_subsampling_y;