6 bit color instead of 9 bit

[fcrovy]

Hi, I'm using your project and I'm very happy, but I want to add some other function (for example a DAC for SAM speech) but there isn't availble pins.
So, the apple 2 manage only 16 colors and can be implemented with 2 bit for any color. In this way 3 pins would be freed up.

What do you think? Is possible without too much effort?

Thank's in advance.,
Fabio

[markadev]

Hey there, I haven't looked into it so it may be possible. You can find the RGB color codes for lores modes here: https://github.com/markadev/AppleII-VGA/blob/main/pico/colors.c#L10-L25. You could try to preview what the result would look like with an RGB color picker and calculating how those would round down into 2 bits instead of 3.

You could also try reducing the DAC to 6bpp (2 bits per color) on an existing VGA card by removing the 2K resistors. The overall brightness might be too low and the colors might look slightly wrong but maybe you can calculate new suitable resistor values for the 2 bit DACs.

The hires and double hires graphics modes are a little more complicated because even in RGB mode there's a small amount of color blending being done between pixels so the extra bit of color depth makes it look a little smoother. When the "smooth hires" mode feature is enabled then there's even more color blending happening so it would probably look really bad. The hires mode colors are defined in a table that is pre-computed by the python scripts generate_hires_rgb_color_patterns.py and generate_hires_ntsc_color_patterns.py. These scripts are where the blending happens so that the Pico code only has to convert the Apple II bit patterns to the RGB values.

[fcrovy]

Thanks for the suggentions. In this moment I'm busy, I'll try as soon as possible.