Video Processors - the scaling process
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As we have seen, the video processor in your system can be presented with a multitude of resolutions. Most signals in the UK will be a PAL 720 x 576 resolution, with high definition 1280 x 720 and 1920 x 1080 rearing it's head in the not too distant future. The problem here is most displays are made up of a different configuration of pixels to this. The processor must increase (upscale) or decrease (downscale) the number of pixels in the image in order to fit. This is not as simple as it would sound. Making an object exactly 2 pixels high by 2 pixels wide, fit a grid of pixels 3 high and 3 wide simply doesn't work (a real-life example of this would be zooming in on a digital photo on your computer, the bigger it goes the more "blocky" it is) So to do this more analysis must take place to ensure that when pixels are added (or removed) the final result looks the same as it did to start with. Complex mathematics (that I barely understand let alone could explain) analyse each pixel, and then a number of pixels surrounding it. Each pixel, and it's relationship to the surrounding pixels, is given a weighting relative to the importance of that pixel within the image. This is analysed in both the original pixel resolution, and also the target location for that pixel in the output resolution. The surrounding pixels for each individual pixel overlap one another, so when expanded to the new output resolution the scaler decides which original pixel the surrounding ones was most highly weighted to, and then applies the colour detail from said pixel.
Simple eh?
As the image above shows, just scaling 2x2 to 3x3 cannot be done without losing something (usually sharpness). Thankfully to a bigger scale it is not such a bad representation, but the theory is true for all images. In the 3x3 box there are essentially four overlapping 4-pixel blocks, each 4-pixel block equating to a single pixel in the original. This would be known as 4-tap scaling, since each pixel analyses four taps around it. Basic processing might use such a simple 4-tap approach, but most of today's advanced processors would use 16 or more taps (the HQV chip uses 1,024!!!!), So taking the top middle pixel in our example; the top-left pixel of our original would dictate that the target pixel should be black however the top-right pixel of the original would want it to be green! The scaler chooses an average, depending on whether it has weighted the green pixel as more or less important than the black one, which depending on how many taps are involved would have a varying success. As with deinterlacing, the more advanced scaler would go on to use such techniques as motion adaptive analysis (studying for movement), or temporal analysis (studying the same pixel over different periods of time) to improve the accuracy of the weightings employed.
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1/6 Signals Your Processor will come accross 2/6 What exactly is an Interlaced or deinterlaced/progressive signal? 3/6 Film Mode Deinterlacing Process 4/6 Video Mode Deinterlacing Process |