A packing

A color space

In most SGI libraries, a single token encodes both color space and packing. For example, VL_PACKING_RGBA_8 is a 32-bit packing in the RGBA color space. For the VL of DIVO/DIVO-DVC and other advanced products, the two parameters are specified separately with different controls: VL_PACKING and VL_COLORSPACE. The color space must be defined with the VL_COLORSPACE control.

In all illustrations, as you move from left to right:

Each illustration shows the smallest repeating spatial pattern of component samples that is a multiple of 8 bits wide. No additional padding or alignment is to be inferred. For example, a 24-bit-per-pixel diagram, such as that for VL_PACKING_444_8 (Figure C-1), indicates 3-byte quantities packed together in memory; the values are not padded out to 32-bit boundaries.

The packing defines a bit layout, but for convenience, as shown in Figure C-1, the component slots are filled with the RGBA or VYUA color set as appropriate. (See “Color Spaces ” later in this appendix for more information.) For chroma components, Cr and Cb are more accurate terms than V and U, because the analog NTSC video specification ANSI/SMPTE 170M uses V and U with a slightly different meaning. However, this appendix uses the letters V and U in the illustrations of packings for typographical convenience.

Packings that use 4:2:2 sampling also show the location of each component sample: left and right for 4:2:2. The diagrams assume row-major, left-to-right ordering of pixels in memory.

The DIVO or DIVO-DVC device can natively transfer data of all the packings shown in this appendix in real time.

Following each packing diagram are comments and library tokens for that packing, listing, where applicable, the color set (RGBA or VYUA) and the library (VL, OpenGL, and DM) for each library token.

In packing tokens, the following applies:

Table C-1 lists the DIVO/DIVO-DVC packings in the order of the number of bits in the pattern of component samples—the order in which they are described in the rest of this section.

The packings are explained in these categories:

Figure C-2 shows the VL_PACKING_4_8, an 8-bit packing useful for VYUA monochrome/luma only.

This packing is

Figure C-3 shows VL_PACKING_R444_332, an 8-bit packing in the RGBA color space.

This packing is

Figure C-4 shows VL_PACKING_444_332, an 8-bit RGBA packing.

This packing is

Figure C-5 shows VL_PACKING_242_8, a 16-bit VYUA packing.

This rarely used packing is VL_PACKING_242_8 + VL_COLORSPACE_{CCIR,YUV} in the VL. It samples chroma and luma in a 4:2:2 pattern. See “Sampling Patterns”, later in this appendix.

Figure C-6 shows VL_PACKING_R242_8, a 16-bit 4:2:2 VYUA packing. The most commonly used 4:2:2 packing, it is used by other SGI video hardware as well as DIVO or DIVO-DVC hardware.

This packing is

Figure C-7 shows VL_PACKING_X4444_5551, a 16-bit RGBA packing that corresponds to the QuickTime file 16-bit uncompressed format with alpha.

This packing is

DM_IMAGE_PACKING_XRGB1555 in DM (even though the upper bit is really alpha)

Figure C-8 shows VL_PACKING_444_5_6_5, a 16-bit RGBA packing.

This packing is VL_PACKING_444_5_6_5 + VL_COLORSPACE_{RGB,RP175} in the VL, new style. 

Figure C-9 shows VL_PACKING_242_10, a 20-bit VYUA packing.

This packing is VL_PACKING_242_10 + VL_COLORSPACE {CCIR,YUV}.

Figure C-10 shows VL_PACKING_R242_10, a 20-bit VYUA packing.

This packing is VL_PACKING_R242_10 + VL_COLORSPACE {CCIR,YUV}.

Figure C-11 shows VL_PACKING_444_8, a 24-bit RGBA/VYUA packing.

This packing is

Figure C-12 shows VL_PACKING_R444_8, a 24-bit RGBA/VYUA packing.

This packing is

Figure C-13 shows VL_PACKING_4444_6, a 24-bit DIVO/DIVO-DVC-only packing with 6 bits per pixel.

This packing is

This section explains

Figure C-26 shows VL_PACKING_444_12, the 36-bit packing, which has 12 bits per component. Only DIVO and DIVO-DVC use this packing.

This packing is

The components in this packing are signed:

Figure C-27 shows VL_PACKING_4444_12, a 48-bit packing, with 12 bits per component. Only DIVO and DIVO-DVC use this packing.

This packing is

The components in this packing are signed:

Figure C-28 shows VL_PACKING_444_10_in_16_L, a 48-bit packing, with 10 bits per component and no alpha. For an explanation of the p bit, see “Packing Diagram Conventions”.

This packing is

Figure C-29 shows VL_PACKING_4444_10_in_16_L. For an explanation of the p bit, see “Packing Diagram Conventions”.

This packing is

Figure C-30 shows VL_PACKING_4444_10_in_16_R.

This packing is

Figure C-31 shows VL_PACKING_4444_12_in_16_L, a 64-bit RGBA packing. For an explanation of the p bit, see “Packing Diagram Conventions”.

This packing is VL_PACKING_4444_12_in_16_L + VL_COLORSPACE_{RGB,RP175} in the VL, new style.

The components in this packing are signed:

Figure C-32 shows VL_PACKING_4444_12_in_16_R, a 64-bit RGBA packing for use with extended RGB components.

This packing is VL_PACKING_4444_12_in_16_R + VL_COLORSPACE_{RGB,RP175} in the VL, new style.

Figure C-33 shows VL_PACKING_4444_13_in_16_L, a 64-bit RGBA packing for use with extended RGB components. For an explanation of the p bit, see “Packing Diagram Conventions”.

This packing is VL_PACKING_4444_13_in_16_L + VL_COLORSPACE_{RGB,RP175} in the VL, new style.

The components in this packing are signed, with positive and negative values varying by color space, as explained for VL_PACKING_4444_12_in_16_L.

Figure C-34 shows VL_PACKING_4444_13_in_16_R, a 64-bit packing for use with extended RGB components.

This packing is VL_PACKING_4444_13_in_16_R + VL_COLORSPACE_{RGB,RP175} in the VL, new style.

Some of the diagrams in the “DIVO/DIVO-DVC Pixel Packings” section indicate 4:4:4 or 4:4:4:4 sampling. This video industry terminology means that each of the three or four components is sampled at every pixel. Figure C-35 diagrams this sampling pattern.

The packings shown in diagrams that indicate 4:2:2 sampling make sense only in the VYUA color spaces. For every two pixels, there are two luma samples (two Ys) but only one chroma sample (one sample of Cr and Cb, which together determine the chroma), as shown in Figure C-36.

The chroma samples belong at the same instant in space as the left Y sample (the chrominance samples and the left Y are co-sited). The diagrams for 4:2:2 packings in the “DIVO/DIVO-DVC Pixel Packings” section of this appendix show the location of each Y, Cr, or Cb component as left or right. The first pixel of each line is a left pixel.

Converting 4:4:4 video to 4:2:2 video is like converting 44.1 kHz audio into 22.05 kHz audio: just dropping every other Cr,Cb sample yields extremely poor results. Video devices that need to convert between 4:4:4 and 4:2:2 use carefully designed filters. The characteristics of the required filter are specified in ITU-R BT.601-4 (Rec. 601).

4:2:2 sampled packings that also include alpha are called 4:2:2:4. This method has one alpha value per pixel, like the Y value.

The packings shown in diagrams that indicate 4:1:1 sampling make sense only in the VYUA color spaces. For every four pixels, there are four luma samples (four Ys) but only one chroma sample (one sample of Cr and Cb, which together determine the chroma), as shown in Figure C-37.

The chroma samples belong at the same instant in space as the left Y sample (the chrominance samples and the left Y are co-sited).

The DIVO-DVC board uses 4:1:1 sampling for 525/60 (NTSC) DV, NTSC DVCPRO, and 625/50 (PAL) DVCPRO compression.

The packings shown in diagrams that indicate 4:2:0 sampling make sense only in the VYUA color spaces. For every two pixels, there are two luma samples (two Ys) but only one chroma sample (on alternate lines, one sample of Cr and one sample of Cb), as shown in Figure C-38.

The chroma samples belong at the same instant in space as the left Y sample (the chrominance samples and the left Y are co-sited).

The DIVO-DVC board uses 4:2:0 sampling for PAL (625/50) DV compression.

For OpenGL, IRIS GL, and DM:

Using the traditional VL_PACKING tokens from IRIX 6.2, the VL_PACKING constant indicates whether the data is RGBA or VYUA (as in VL_PACKING_UYV_8_P). The VL that comes with the DIVO or DIVO-DVC option (for IRIX 6.4 and later) makes all of the parameters (packing, set of colors, range of components) explicit:

The DIVO and DIVO-DVC options perform color-space conversion if the color space implied by VL_FORMAT on the video node disagrees with that implied by VL_COLORSPACE on the memory node.