--- title: "Color parameter atom ('colr')" framework: quicktime-file-format role: symbol role_heading: Atom path: quicktime-file-format/color_parameter_atom --- # Color parameter atom ('colr') A required extension for uncompressed Y´CbCr data formats. ## Mentioned in QuickTime File Format change log ## Overview Overview This atom is a required extension for uncompressed Y´CbCr data formats. The 'colr' extension is used to map the numerical values of pixels in the file to a common representation of color in which images can be correctly compared, combined, and displayed. The common representation is the CIE XYZ tristimulus values (defined in Publication CIE No. 15.2). Use of a common representation also allows you to correctly map between Y´CbCr and RGB color spaces and to correctly compensate for gamma on different systems. The 'colr' extension supersedes the previously defined 'gama' Image Description extension. Writers of QuickTime files should never write both into an Image Description, and readers of QuickTime files should ignore 'gama' if 'colr' is present. The 'colr' extension is designed to work for multiple imaging applications such as video and print. Each application, driven by its own set of historical and economic realities, has its own set of parameters needed to map from pixel values to CIE XYZ. The CIE XYZ representation is mapped to various stored Y´CbCr formats using a common set of transfer functions and matrixes. The transfer function coefficients and matrix values are stored as indexes into a table of canonical references. This provides support for multiple video systems while limiting the scope of possible values to a set of recognized standards. The 'colr' atom contains four fields: a color parameter type and three indexes. The indexes are to a table of primaries, a table of transfer function coefficients, and a table of matrixes. The following table shows the layout of this atom. | | | | | | | QuickTime uses the transfer function and matrix to convert RGB into Y´CbCr and back, as the following diagram shows: The Y´CbCr values stored in a file are normalized to a range of [0,1] for Y´ and [-0.5, +0.5] for Cb and Cr when performing these operations. The normalized values are then scaled to the proper bit depth for a particular Y´CbCr format before storage in the file as follows: note: The symbols used for these values are not intended to correspond to the use of these same symbols in other standards. In particular, “E” should not be interpreted as voltage. These normalized values can be mapped onto the stored integer values of a particular compression type’s Y´, Cb, and Cr components using two different schemes, which we will call Scheme A and Scheme B. warning: Other, slightly different encoding/mapping schemes exist in the video industry, and data encoded using these schemes must be converted to one of the QuickTime schemes defined here. Scheme A uses “Wide-Range” mapping (full scale) with unsigned Y´ and twos-complement Cb and Cr values as the following figure shows: This maps normalized values to stored values so that, for example, 8-bit unsigned values for Y´ go from 0-255 as the normalized value goes from 0 to 1, and 8-bit signed valued for Cb and Cr go from -127 to +127 as the normalized 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 symbols Cb and Cr are used to describe offset binary integers, not twos-complement signed integers shown here. Scheme B uses “Video-Range” mapping with unsigned Y´ and offset binary Cb and Cr values. note: Scheme B, shown in the following figure, comes from digital video industry specifications such 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 SGI also use this scheme. MPEG-2, DVC and many other codecs specify source Y´CbCr pixels using this scheme. This maps the normalized values to stored values so that, for example, 8-bit unsigned values for Y´ go from 16 to 235 as the normalized value goes from 0 to 1, and 8-bit unsigned valued for Cb and Cr go from 16 to 240 as the 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 have the range of 65–960 as the normalized values go from –0.5 to +0.5. Y´ is an unsigned integer. Cb and Cr are offset binary integers. Certain Y´, Cb, and Cr component values v are reserved as synchronization signals and must not appear in a buffer. For n = 8 bits, these are values 0 and 255. For n = 10 bits, these are values 0, 1, 2, 3, 1020, 1021, 1022, and 1023. The writer of a QuickTime image is responsible for omitting these values. The reader of a QuickTime image may assume that they are not present. The remaining component values that fall outside the mapping for scheme B (1 to 15 and 241 to 254 for n = 8 bits and 4 to 63 and 961 to 1019 for n = 10 bits) accommodate occasional filter undershoot and overshoot in image processing. In some applications, these values are used to carry other information (e.g., transparency). The writer of a QuickTime image may use these values and the reader of a QuickTime image must expect these values. The following tables show the primary values, transfer functions, and matrixes indicated by the index entries in the 'colr' atom. The R, G, and B values in the following list are tristimulus values (such as candelas/meter^2), whose relationship to CIE XYZ values can be derived from the primaries and white point specified in the table, using the method described in SMPTE RP 177-1993. In this instance, the R, G, and B values are normalized to the range [0,1]. The transfer functions listed are used to transfer between RGB and Y’CbCr color spaces. The MPEG-2 sequence display extension transfer_characteristics defines a code 6 whose transfer function is identical to that in code 1. QuickTime writers should map 6 to 1 when converting from transfer_characteristics to transferFunction. Recommendation ITU-R BT.470-4 specified an “assumed gamma value of the receiver for which the primary signals are pre-corrected” as 2.2 for NTSC and 2.8 for PAL systems. This information is both incomplete and obsolete. Modern 525- and 625-line digital and NTSC/PAL systems use the transfer function with code 1. The matrix values are shown in the following list and in Matrix values for index code 1. These figures show a formula for obtaining the normalized value of Y´ in the range [0,1]. You can derive the formula for normalized values of Cb and Cr as follows: If the equation for normalized Y´ has the form: Then the formulas for normalized Cb and Cr are: ## Topics ### Data fields - [Size](quicktime-file-format/color_parameter_atom/size.md) - [Type](quicktime-file-format/color_parameter_atom/type.md) - [Color parameter type](quicktime-file-format/color_parameter_atom/color_parameter_type.md) - [Primaries index](quicktime-file-format/color_parameter_atom/primaries_index.md) - [Transfer function index](quicktime-file-format/color_parameter_atom/transfer_function_index.md) - [Matrix index](quicktime-file-format/color_parameter_atom/matrix_index.md) ## See Also ### Extending video sample descriptions - [Pixel aspect ratio ('pasp')](quicktime-file-format/pixel_aspect_ratio.md) - [MPEG-4 elementary stream descriptor atom ('esds')](quicktime-file-format/mpeg-4_elementary_stream_descriptor_atom.md) - [AVC decoder configuration atom ('avcC')](quicktime-file-format/avc_decoder_configuration_atom.md) - [Clean aperture ('clap')](quicktime-file-format/clean_aperture.md)