Originally posted by: Jeff7
I guess I'm still not clear on this lower resolution interpolation thing either. Why isn't it possible to make an LCD with the same pixel layout as a CRT? It's not like a CRT has an infinite number of holes in the aperature grille or shadow mask.
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Because a CRT's pixels aren't fixed in size and grow/shrink as required by the resolution. An LCD is just basically a bunch of lights that turn on and off but can never change size.
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I don't get this either; I thought a pixel is fixed size no matter what, unless my idea of a screen pixel is incorrect. I was under the impression that a pixel is a group of three holes, with red, green, and blue phosphors.
Yes, CRTs have a fixed number of screen phosphors -- but they way they 'interpolate' the pixels is very different from a fixed-pixel display (sort of a misleading name; better might be 'directly addressable' or something like that).
A CRT display is
analog -- when you tell it to run, say, 800x600, it just scans the electron beam across the screen at a certain rate, changing the intensity so that it has 800 different values evenly spaced horizontally, and 600 distinct values evenly spaced vertically. The output of the electron beam is actually a continuous analog waveform. Each phosphor effectively takes a *slightly* blurry sample of the electron beam value at its location. What this means is that if you increase the resolution, the samples get more precise -- and because they're oh-so-slightly blurry analog samples of a continuous waveform, it always does a pretty good job of approximating it. There are no values of horizontal and vertical frequency for which it is noticeably better or worse.
LCD displays are 100% digital. They can *only* display images in one fixed resolution, and the electronics in the screen tell each pixel *exactly* what to display. So, if you feed the display a video signal that matches up exactly with the 'native' resolution (the actual output pixels), it can do a 1:1 mapping of input pixels to output pixels, and everything works great. In fact, you probably get a more 'accurate' picture than on a CRT, objectively speaking. The problem arises when you can no longer do a 1:1 mapping -- if the input resolution is higher or lower than the output. If it is lower, you have two choices -- you can either scale the input to match the output, or you can display it without scaling, only using some of the pixels of the display (this maintains sharpness, but gives a smaller overall image). If it is higher than the native resolution, obviously you have to scale it (or else you would only display part of the image).
The problems with scaling are mostly mathematical. Unless the output resolution is an exact multiple of the input resolution, you have to resample the image in some way. This means that at least some of the output pixels correspond to more than one input pixel. But because this is a digital system (often with a very limited 6-bit color resolution), this results in a slight blurriness, especially in areas of high contrast. With the analog samples of a CRT's phosphors, you have an effectively infinite range of possible color values, which produces smoother results (at least to our eyes). Also, a CRT's phosphors are generally more numerous and
more tightly packed than an LCD's pixels, meaning at least some of the samples will be closer to the 'actual' locations of the pixel centers at any resolution. With a fixed-pixel display, some resolutions will result in situations where some pixels will be 'between' the display's actual pixels. Compounding this is the fact that there are multiple ways to *do* the scaling (various types of linear interpolation, gaussian sampling, etc.), some of which are better or worse than others (take a look at Photoshop and try using the different resize options to get an idea of what I'm talking about). The better ones tend to be quite computationally intensive, and can be hard to do in real time.
So, basically, if you made an LCD screen with a resolution comparable to the spacing of a CRT monitor's phosphors, you should get results comparable to a CRT. Increasing the color depth (up to and beyond 8 bits per pixel) would help as well, since the interpolation would be more accurate. And there may still be room for improvement in terms of how the scaling is actually done.
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