New Graphics modes
Quite a lot of interest was recently shown when I asked if anyone would be interested in additional graphics modes for the AppleSqueezer GS Turbo. So, I added some new modes, which I think offer a lot of possibilities. The Apple IIGS of course already has a bunch of modes, which all output to the analog RGB output on the back of the machine, suitable for a CRT colour monitor. The new modes cannot alter or add to these modes as they are fixed to the chips on the IIGS motherboard. However, new modes can be added to the HDMI output, so this is exactly what I've done.
Keep in mind that, because the new modes only work through HDMI, using them limits your audience to people who use the HDMI output! Therefore, it's a good idea to always have a fallback option for people who do not use HDMI, or for those who do not have an AppleSqueezer GS Turbo. It depends on your software if you want to do this or not, but you should at least keep it in mind. For certain new games for example, it may not make sense to have a fallback option, because the new graphics modes will allow for gameplay that is simply not possible through the existing modes.
Ok, with all that in mind, let me give a few reasons for the new modes:
They offer higher resolution, and square pixels. One drawback of the highest resolution (640x200) SHR mode on the IIGS is that its pixels are elongated, rather than square. Some argue that this alone limited the potential for professional software early in the life of the IIGS, when compared to e.g. the Macintosh, which did have square pixels, even though they were only in black and white initially. Particularly for desktop publishing, this is a severe limitation. So, some of the new modes offer 640x400 and even 640x480, with square pixels. They are otherwise the same as the existing modes, which helps with QuickDraw II compatibility, and should make it relatively easier to create a new GS/OS with 640x400 (or 480) resolution.
They offer a way out of the 1 MHz display bottleneck. Currently, with the CPU in the AppleSqueezer GS Turbo running at 48 MHz, the 1 MHz bus for display graphics can be quite a bottleneck: it essentially limits the maximum frames per second rate to 30 FPS at 640x200 and 320x200 for games that redraw the entire screen, like many scrolling (e.g. platform) games. The new modes work directly with the memory on the AppleSqueezer GS Turbo, so aren't limited to this, and can therefore reach a much higher FPS. It's relatively simple to create games with a full 60 FPS through these new modes!
There is a new mode especially aimed at games: 320x200 or 320x240, with 256 colours on screen at once, each chosen from 16.7 million. Not only is this very colourful, but this mode also offers hardware scrolling, which allows for all sorts of games like platform games at 60 FPS with more than 100 sprites.
Page flipping: you can draw the next picture in another bank and switch at the start of a frame. So, there's then no tearing, essential for smooth animation.
The 256-colour mode also offer per-line offsets: parallax scrolling, bands of the picture moving at different speeds, as on the SNES. The demo code that's available shows how this works, and it's described in the Programmer's Guide.
I marked the new graphics modes as experimental, for the time being. The FPGA is now quite full so there's not much that can be added. And that may not be necessary, but should there be more important things to add later, then I may remove them again. It also depends on whether there will be software taking advantage of the new modes of course!
The new modes are available as part of Core 27, which I just released. And to take advantage of the modes, you can read all the technical details, and examples, in the updated AppleSqueezer Programmer's Guide. When updating your AppleSqueezer GS Turbo, don't forget to use the new CDev, not the old v2 CDev!! (if you do, you break SD card functionality - which can be fixed with the help of a fixer program, see the FAQ)
There is also some test software available, you can load this .po file onto an SD card, plug it into your Turbo, and view several examples of the new modes. This software, and its source, is available in the Downloads section.
I hope to see a lot of games or other experiments that take advantage of the new modes!
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