Quicktime File Format (2012-08-14).pdf

Media Data Atom TypesVideo MediaScheme A uses "Wide-Range" mapping (full scale) with unsigned Y´ and twos-complement Cb and Cr valuesas shown in Figure 4-4 (page 164).Figure 4-4Equations for stored Y´CbCr values of bit-depth of n in scheme AThis maps normalized values to stored values so that, for example, 8-bit unsigned values for Y´ go from 0-255as the normalized value goes from 0 to 1, and 8-bit signed valued for Cb and Cr go from -127 to +127 as thenormalized values go from -0.5 to +0.5.Warning In specifications such as ITU-R BT.601-4, JFIF 1.02, and SPIFF (Rec. ITU-T T.84), the symbolsCb and Cr are used to describe offset binary integers, not twos-complement signed integers shownhere.Scheme B uses "Video-Range" mapping with unsigned Y´ and offset binary Cb and Cr values.Note Scheme B, shown in Figure 4-5 (page 164), comes from digital video industry specificationssuch as Rec. ITU-R BT. 601-4. All standard digital video tape formats (e.g., SMPTE D-1, SMPTE D-5)and all standard digital video links (e.g., SMPTE 259M-1997 serial digital video) use this scheme.Professional video storage and processing equipment from vendors such as Abekas, Accom, and SGIalso use this scheme. MPEG-2, DVC and many other codecs specify source Y´CbCr pixels using thisscheme.Figure 4-5Equations for stored Y´CbCr values of bit-depth n in scheme BThis maps the normalized values to stored values so that, for example, 8-bit unsigned values for Y´ go from 16to 235 as the normalized value goes from 0 to1, and 8-bit unsigned valued for Cb and Cr go from 16 to 240 asthe normalized values go from -0.5 to +0.5.For 10-bit samples, Y´ has a range of 64 to 940 as the normalized value goes from 0 to 1, and Cb and Cr havethe range of 65–960 as the normalized values go from –0.5 to +0.5.2012-08-14 | © 2004, 2012 Apple Inc. All Rights Reserved.164