I know I’ve said this before, but I think BMOW is fast approaching a state of “done”. For the last week I’ve been polishing some old demos and making some new ones, for inclusion in the final demo ROM. Along the way, I also added a proper startup sound. Turn the machine on, and it announces itself to the world, loud and proud.

I also revisited the video hardware, and created a couple of new demos showing “new” video modes. These modes aren’t really new, of course. The hardware capability was always there, and I knew the modes were theoretically possible, but I’d never before actually taken the time to determine the necessary combination of register settings and video data interleaving to draw an image in every mode. My first target was the highest resolution mode possible for BMOW’s video system: 512×480, with two colors. It is truly any two colors, and not black and white, as this brown and tan Aztec Sun Stone image shows. Click the image to see a full-size version with greater detail.

I then created a second demo that makes use of mixed mode, with the screen split into three regions in 2 bits-per-pixel, 4 bits-per-pixel, and 8 bits-per-pixel modes. The same image of a flowering hedge is shown in all three regions, to demonstrate how the BMOW hardware trades color depth for horizontal resolution.

The 2bpp sample was a bit of a challenge, and was another mode I’d never actually tried before. It turned out that my image editor (GIMP) doesn’t really output 2bpp images, with 4 pixels packed per byte. Instead, it outputs an image in 4bpp format, that just happens to have only 4 colors in its palette instead of 16. Rather than hack the image data, I wrote a BMOW conversion routine to repack the 2bpp image data before it’s displayed.

The screenshot below shows the comparison of the three color depths, but to really appreciate the differences, click the image to see a higher-res screenshot. The 256 color region has rich, green, shiny leaves and a buttery yellow flower, but looks like a mosaic due to the low resolution. The 16 color region holds up well, given that most of the image is green anyway, but the flower becomes a solid yellow region. The flower turns green in the 4 color region, but thanks to the higher resolution and effective dithering, the leaves don’t look too bad.

If I’d chosen an image with a greater variety of colors, the discrepancy between the three would have been much more obvious. A grayscale palette test might be interesting too.