[aardvark179]

That particular problem isn't a case of emulating the television, but rather accurately emulating the console's video hardware and its interactions with the rest of the system. If one were simply interested in emulating the television's behaviour then you could construct a frame buffer based on the visible sprites and postprocess that (possibly in conjunction with several preceding fields).

If the console allowed sneaky things to be done on each raster line (like changing the colours) then constructing that frame buffer becomes considerably more resource intensive, as it must now probably be done line by line with the correct timing wrt. the rest of the emulation.

If you could pull tricks mid scanline (presumably through careful timing after an interrupt) then the problem will be a whole lot worse, though I'd guess it can be reduced by recording changes to the relevant hardware registers along with timestamps in the emulation so that the timing of your scanlines' construction becomes less of an issue.

[T-hawk]

You're correct, this particular problem can be handled with sufficiently sophisticated frame buffer logic. I was generalizing from that to other concepts where emulating the television or its signal processing would be required.

I'll give you another example. On the Atari 2600 game console, the vertical sync is software controlled. The software is responsible for enabling the vertical sync pulse. This can be done 60 times per second as standard -- or you could play tricks with it. Suppose you strobe it at a different or even irregular rate. On an analog TV, the picture starts rolling vertically. That breaks way outside the sandbox of a framebuffer, with signal being displayed in overscan areas, and during the normally-blank retrace interval resulting in ghosting effects. (No commercial game did that, but it's been done in tech demos, and conceivably a horror game could do it intentionally for mood.) To produce that same behavior on framebuffer-based hardware, you need to emulate or at least approximate the workings of a TV's vertical sync logic, none of which appears in the console itself.

(I know this from experience, I wrote an Atari 2600 game: http://www.dos486.com/atari/ )

[Keyframe]

I recently bought 'Racing the beam' book which goes into Atari 2600 VCS details and programming - http://mitpress.mit.edu/catalog/item/default.asp?ttype=2&... I recommend this book to everyone interested in this topic.

[byuu]

> I'd guess it can be reduced by recording changes to the relevant hardware registers along with timestamps in the emulation so that the timing of your scanlines' construction becomes less of an issue.

This would be possible in most cases, but the SNES throws another problem at you: the video renderer can set flags that can affect the operation of the CPU. Range/tile over sprite flags, H/Vblank signals, etc.

In my model, I chose to forgo timestamps because they are very tricky to get right with subtle details. Instead, I render one pixel at a time, but I use a cooperative threading model. Whenever the CPU reads something from the PPU, it checks to see if the CPU is currently not caught up with the PPU. If so, it will switch and run the PPU. The PPU does the same with respect to the CPU.

Even with that, all the extra overhead of being -able- to process one pixel at a time knocks the framerate from ~240fps to ~100fps. And it fixes maybe a half-dozen minor issues in games for all that overhead.

This is because scanline-based renderers are notoriously good at working around these issues. There are lots of games that do a mid-scanline write in error, but only a few that do more than one on the same scanline. So all you have to do is make sure you render your line on the correct 'side' of the write. We actually took every game we could find with this issue, and averaged out the best possible position within a line to run the highest number of games correctly. Other emulators take that further and can make changes to that timing on a per-game basis to fix even more issues.

Air Strike Patrol's shadow is actually the only known effect where two writes occur on the same line, and there is no one point that would render the line correctly.