Lost in Translation: Going from TV to PC

Of particular concern is the point where RGB combine to form white (see Figure 5, right). All whites are not identical; in video, "whiteness" is expressed as a color temperature, with 6,500K considered daylight. Values below 6,500 become more orange; those above become bluer. A display that is mismatched for color temperature will produce improper colors.

Gamma Correction
The response to an input signal by a display, known as the transfer function, can vary. A linear relationship is ideal, but it is rarely the case. In CRTs the relationship is exponential. This characteristic of the relationship of display output to the stimulus voltage level is referred to as the gamma. The equation (for R or any luminance of color signal) is shown in Figure 6.

Figure 6. Gamma conversion equation (shown for R)

Gamma curves vary depending on display technology. Figure 7 shows a pair of complementary gamma values that when applied at the encoder and decoder result in an ideal gamma of 1.0. If the gamma values are not perfectly complementary, image color will not match the source, and picture fidelity will suffer.

Figure 7. Clockwise, from top left: Gamma curves for a given value; a complementary pair; output response and correction curves; LCD versus CRT gammas

Scaling Issues
DTV formats are not a perfect fit for the native resolution of PC displays. HD formats are defined as pixel grids of 1920 horizontal by 1080 vertical elements or 1280x720. Computer displays come in a wide variety of native resolutions. Aspect ratios of 4:3 and 16:9 are common.

Even very expensive professional graphics monitors are rarely configured for true HD pixel grids, a 1920x1200 configuration often being the case. This does not conform to the typical HD ratio of 16:9; as a result, a DTV image is often letterboxed with black bars at the top and bottom of the display.

But the real problem lies in pixel-grid conversion. Even if the accumulation of errors to this point is imperceptible, the amount of damage that will be done in this stage is frequently a showstopper. Figure 8 is a visual explanation of what goes wrong.

Figure 8. Scaling issues for pixel-grid conversion between 1080 and 720-line resolutions

Consider a conversion between pixels that have a 3:2 relationship. As Figure 8 shows, when the source image consists of alternating sets of three black pixels and three white pixels, the conversion is trivial and artifact-free. In Figure 8, the image source now consists of alternating black and white pixels (this is the killer test pattern), and conversion is no longer so simple. For every pixel that aligns, the next three do not. Consequently, that pixel must be interpolated, frequently using an averaging algorithm, resulting in gray pixels. Figure 8 illustrates the problem when an image in upscaled.

Here’s a simple test you can try on your PC. Open any program that has drawing capabilities; Microsoft Word will do just fine. Draw a box of any size and fill it with black. Now draw a line across it and change it to white. So far so good; the white line is still white. Now move the line. It turns a shade of gray. Try it again and you get the same result. Draw another box and white lines but don’t move them. Now group them and resize the box. Again, the lines turn gray.

The good news is that real-world images rarely consist of high-contrast detail at the extremes of the imaging scale. The bad news is that scaling will always produce some amount of image degradation.

Pixel Squareness
The relationship between the dimensions of the grid and the aspect ratio is described as pixel squareness. For a pixel to be considered square, the ratio of horizontal to vertical pixel-grid dimensions must be the same as the display. For example, a 640x480 VGA display on a 4:3 aspect ratio display is said to have square pixels. HDTV 1080i, 1080p, and 720p formats all have square pixels. However, a standard-definition digital TV pixel grid is defined as 720x480 and is not square for obvious reasons. As a result, converting between source image and displayed image is not always simple one-to-one pixel mapping.

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