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Re: CGA Composite Mode under DOSBOX?

Posted on 2012-04-20, 15:15

If we're doing our band-limiting by just averaging over 4 consecutive pixels (as the current DOSBox implementation does, I think) and we limit ourselves to graphics modes we can just enumerate all the colour possibilities directly in the palette. There are only 64 of them Having thought about this …

Re: CGA Composite Mode under DOSBOX?

Posted on 2012-04-19, 22:51

A gated and filtered signal derived from subcarrier, called the burst, shall be added in the horizontal blanking interval of each line, excluding the nine-line vertical sync interval, as a synchronizing signal and amplitude reference for the chrominance signals. Now you've got me wondering if I was …

Re: CGA Composite Mode under DOSBOX?

Posted on 2012-04-19, 20:51

In any case, the CGA does not generate proper video levels, and having gone back and forth between several ways of dealing with this, I think for emulation purposes, the best way is to just normalize everything so that black really is at 0/0/0 and white is at 1/1/1. Agreed. I still want to figure …

Re: CGA Composite Mode under DOSBOX?

Posted on 2012-04-19, 20:39

As Qbix noted, the precalculation produces 80 colors (16 hues * 5 luminance levels for each); the line-drawing handler then determines the palette index for each pixel by doing some averaging over adjacent pixels. Having thought about this some more - I think there's a slight variation on this …

Re: CGA Composite Mode under DOSBOX?

Posted on 2012-04-19, 19:18

As it should. If your purpose of performing the band-limiting is to replicate the blur on a monochrome monitor, then you would be right, and you would have to linearize the signals before applying a linear transformation. But when the NTSC standard was first developed, that was exactly the idea - …

Re: CGA Composite Mode under DOSBOX?

Posted on 2012-04-19, 16:48

No, that's entirely correct, because YIQ to RGB transformation as well as the band-limiting is even in theory meant to apply to electrical signals, not to light. I'm not convinced about that - it's a linear transformation of voltage but ideally it would be linear transformation of light. Consider a …

Re: CGA Composite Mode under DOSBOX?

Posted on 2012-04-19, 04:02

The "non-linearity" merely refers to what the electrical (or digital) RGB signals actually represent. A linear signal directly represents light, because double the voltage means double the light, whereas a "non-linear" signal doesn't directly represent light, because it's "distorted", if you will, …

Re: CGA Composite Mode under DOSBOX?

Posted on 2012-04-18, 22:00

As for the hard part (refactoring the proposed code to work with DOSBox's palette precalculation), here's what I was thinking: derive the 16 pixel patterns corresponding to each nybble for the current mode/color register, and feed them to reenigne's code (upsampled as necessary - the algorithm …

Re: CGA Composite Mode under DOSBOX?

Posted on 2012-04-18, 21:31

Most CRT television sets don't convert to linear RGB, as that is difficult with analogue electronics, but just apply the conversion matrix to the non-linear voltages from the YIQ decoder. Ah, here's where the confusion lies then - my understanding is that the voltages (composite, Y, I, Q and the R, …

Re: CGA Composite Mode under DOSBOX?

Posted on 2012-04-18, 18:57

A previous statement of mine warrants a correction: NTSC assumes a color space vastly different than today's sRGB monitors (see NTSC Wikipedia article ). Thus, after producing RGB values using the regular YIQ-to-RGB conversion matrix (as used in DosBox' implementation), these RGB values then need …

Re: CGA Composite Mode under DOSBOX?

Posted on 2012-04-18, 17:20

That hue offset is emulating a particular clone behaviour around an unused bit. KQ1 booter benefits from it. Oh yes, I remember that thread now that you mention it. I just tracked it down and http://www.vogons.org/viewtopic.php?t=9765&postdays=0&postorder=asc&start=260 seems to be the relevant page …

Re: CGA Composite Mode under DOSBOX?

Posted on 2012-04-18, 15:47

could a new vs old cga be a reason for why some games need hue changed by 15 degrees? The hues of both chroma and artifact colours should be the same between old and new CGA - only the brightnesses and saturations should be different. The hues of some of the colours formed by the interaction …

Re: CGA Composite Mode under DOSBOX?

Posted on 2012-04-17, 19:01

If this is accurate, it would seem not: You're confusing two things here - the BIOS mode 6 composite artifact colours (two of which are identical greys, bit patterns 0101 and 1010, as a result of the way that artifact colours are generated), and the "chroma colours" which are the solid colours …

Re: CGA Composite Mode under DOSBOX?

Posted on 2012-04-17, 14:58

Not sure if it'll make any difference, but my vote is for a new machine type or other config switch. My reasoning is that composite isn't a "mode" of the CGA card, it's a completely different type of monitor (which connects to a different port on the back of the CGA card). All modes can display on …

Re: CGA Composite Mode under DOSBOX?

Posted on 2012-04-08, 23:46

NewRisingSun's old post ( http://www.vogons.org/viewtopic.p...ight=66515 ) indicates that he was using wide-angle color demodulation for his filtering, I read through the thread (again!) and came to these conclusions: * Wide-angle color demodulation is a hack which some TVs implemented to make the …

Re: CGA Composite Mode under DOSBOX?

Posted on 2012-04-06, 15:57

When using a 135 degree offset to test Tandy 1000 colors, I noticed that the chroma colors get tinted as well, though they're not supposed to. This in turn affects the artifact colors and overall the results look wrong. Oh yes, you'll have to use a different color_burst array for Tandy 1000 as well …

Re: CGA Composite Mode under DOSBOX?

Posted on 2012-04-06, 00:38

My multi-norm TV does the vertical filtering even when fed NTSC, and I personally think Mask of the Sun and other games that use this horizonzal striping look better that way. Can you take a picture of what those same Mask of the Sun screens look like on your TV? I'm interested to see what it makes …

Re: CGA Composite Mode under DOSBOX?

Posted on 2012-04-05, 19:55

Looking at Servo's hardware captures on Mobygames (see this one from the same game), they seem to have more advanced filtering going on, with less "fringing"; but I can only assume that something like this would be too computationally costly to apply in real-time, e.g. for DOSBox. , Actually I …

Re: CGA Composite Mode under DOSBOX?

Posted on 2012-04-05, 04:59

Does a comb filter enter into the consideration here at all? I thought its job was to "comb out" the relatively narrow bands introduced by the color signal from the upper range of the luminance signal. This would increase horizontal sharpness but wouldn't affect the vertical at all, or would it? …

Re: CGA Composite Mode under DOSBOX?

Posted on 2012-04-04, 04:58

It's curious - the images produced seem much "blockier" than I was expecting, as if they were rendered at 320x200. I think the code is correct though - it's just than I'm using a much less sharp FIR filter than NewRisingSun did (mine is based on the one from the xanalogtv program which is part of …

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