The VES Handbook of Visual Effects: Industry Standard VFX Practices and Procedures by Jeffrey A. Okun & Susan Zwerman

Author:Jeffrey A. Okun & Susan Zwerman [Jeffrey A. Okun and Susan Zwerman]
Language: eng
Format: epub
ISBN: 9781136136214
Publisher: Taylor and Francis
Published: 2014-07-10T16:00:00+00:00

Figure 6.6 Top-left , a low-resolution RGB image used for illustrating the use of chroma sub-sampling in compressing images. Top-right , reconstructed RGB image, and the difference between original and reconstructed pixels. Bottom-left , luminance of the original image. Bottom-right , half-resolution chroma image.

(Image courtesy of Florian Kainz.)

The top-left image in Figure 6.6 shows an example RGB image. The image is disassembled into a luminance-only or grayscale image and a chroma image without any brightness information. Next, the chroma image is reduced to half its original width and height. The result can be seen in the bottom images in Figure 6.6 .

Scaling the chroma image back to its original size and combining it with the luminance produces the top-right image in Figure 6.6 . Even though resizing the chroma image has discarded three-quarters of the color information, the reconstructed image is visually indistinguishable from the original. The difference between the original and the reconstructed image becomes visible only when one is subtracted from the other, as shown in the inset rectangle. The contrast of the inset image has been enhanced to make the differences more visible.

The specifics of converting a red–green–blue or RGB image into luminance and chroma components differ among image file formats. Usually the luminance of an RGB pixel is simply a weighted sum of the pixel’s R, G, and B components. Chroma has two components, one that indicates if the pixel is more red or more green and one that indicates if the pixel is more yellow or more blue. The chroma values contain no luminance information. If two pixels have the same hue and saturation, they have the same chroma values, even if one pixel is brighter than the other (Poynton, 2003 and ITU-R BT.709–3).

Converting an image from RGB to the luminance-chroma format does not directly reduce its size. Both the original and the luminance-chroma image contain three values per pixel. However, since the chroma components contain no luminance information, they can be resized to half their original width and height without noticeably affecting the look of the image. An image consisting of the full-resolution luminance combined with the resized chroma components occupies only half as much space as the original RGB pixels. Variations of luminance-chroma encoding are a part of practically all image file formats that employ lossy image compression, as well as most video formats.

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