BitJazz: Products: SheerVideo: Test Data: Y'CbCr 8bv 4:4:4p Data

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SheerVideo Test Data
Y'CbCr 8bv 4:4:4p Data

Y'C_BC_R 8bu/v 4:4:4 formats:
Y'C_BC_R 8bv 4:4:4 is a popular video editing format. The Y'C_BC_R 8bv 4:4:4 format contains three 8-bit video-range values per pixel: one each for

(Y') luma
(C_B) blue-yellow chroma
(C_R) red-cyan chroma

The luma component has a nominal range of 220 unsigned integer values, and the chroma components have nominal ranges of 225 offset binary integer values. The chroma range of [16..240]->[-1/2..1/2] leaves room on both ends for out-of-range colors. Similarly, the luma component's range of [16..235]->[0..1] leaves both footroom for subblack colors and headroom for superwhite colors. These ranges comply with Recommendation BT.601-4 of the International Telecommunication Union.

In the variant Y'C_BC_R 8bu 4:4:4 format, the chroma components are also 601-compliant, but the luma component has an unbiassed range of [0..219], leaving more headroom but no footroom.

Headroom and footroom are useful for filter slop during editing. However, they are by definition not representable as distinct colors on television monitors or computer displays, and must be limited to the nominal range before they can be viewed. Moreover, leaving headroom and footroom reduces the number of distinct displayable colors.

Note that SheerVideo also supports the corresponding Y'C_BC_RA 8bv 4:4:4:4 and Y'C_BC_R 8bu 4:4:4:4 formats, which contain an 8-bit alpha (A) component at each pixel in addition to the luma and chroma components. The alpha component, used during blue-screening and other compositing techniques, specifies how visible or how opaque the pixel is. In the 601-compliant Y'C_BC_RA 8bv 4:4:4:4 format, the alpha component has the same standard video range as the luma component: [16..235]->[0..1]. The Y'C_BC_RA 8bu 4:4:4:4 format has an 8-bit full-range alpha component, [0..255]->[0..1].

QuickTime 6 doesn't include an encoder for any of these formats, so to convert the RGB 8b footage to Y'C_BC_R 8bu/v 4:4:4, we wrote our own codec to translate between the RGB and Y'C_BC_R color spaces using the equations published in BT.601-4.

At three 8-bit components per pixel, the uncompressed size of each of these Y'C_BC_RA 8bv 4:4:4 frames is 3 x 768 x 512 = 1,179,648 bytes. Note that on the Macintosh, an uncompressed Y'C_BC_R 8bu/v 4:4:4 image is actually always represented with 4-byte pixels, as if it were Y'C_BC_RA 8bu/v 4:4:4:4, even though the 4th byte is unused. Thus to compare SheerVideo Y'C_BC_R 8bv 4:4:4 images to actual uncompressed Y'C_BC_R 8bu/v 4:4:4 images on the Mac, multiply the uncompressed sizes and the compression powers listed here by 4/3.

Inside QuickTime, the Y'C_BC_R 8bv 4:4:4 and Y'C_BC_RA 8bv 4:4:4:4 formats are known as 'v408' or k444YpCbCr8CodecType, with the components in the order CbY'CrA. Y'C_BC_R 8bu 4:4:4 and Y'C_BC_RA 8bu 4:4:4:4 are known as 'r408' or k4444YpCbCrA8RCodecType, with the components in the order AY'CbCr. Apple's JPEG encoder supports Y'C_BC_R 8bu 4:4:4 input. Apple's NTSC DV-25 codec supports both Y'C_BC_R 8bv 4:4:4 and Y'C_BC_R 8bu 4:4:4 for input as well as output.

By losing a couple of bits or more of image information per pixel through round-off errors, the conversion from RGB 8b to Y'C_BC_R 8b inevitably seriously degrades the image quality. But the video world tolerates this degradation because that's how color television works. And color television works that way because, when it was being invented in 1950, the National Television Standards Committee (NTSC), in defiance of the Federal Communications Commission (FCC), had the marketing foresight to make it compatible with monochrome television, which only uses the luma channel.