Mono CGA Palette Definition and Color Conversion

[gm1168]

I am attempting to add my own palettes to the current ones (Green, Amber, Gray, White) and while I was able to add one, the colors used were not what I expected. It appears I am using an incorrect conversion to get values in the mono_cga_palettes array (which are then used by VGA_DAC_SetEntry).

Can anyone share the formula to convert standard RGB color codes (e.g. #FFFFFF for black, #FFFF00 for yellow) into the {0x00,0x00,0x00} format used in mono_cga_palettes and VGA_DAC_SetEntry?

I tried an 8bit to 6bit conversion and the results did not look at all like the colors I expected.

[gm1168]

After some further experimenting, it looks like youjust need to perform a left bitshift of two places to convert each byte of standard color code into the {0x00,0x00,0x00} format.

[maron2000]

#FFFFFF is (0xFF, 0xFF, 0xFF) 8bits/color =24bits
Values for CGA pallete (and VGA pallete as well) is 6bits/color, so you need to shift 2 bits for each color, which results in (0x3F, 0x3F, 0x3F).

You should see the palette value for white in the code tree as below.

VGA_DAC_SetEntry(0xf,0x3f,0x3f,0x3f);

Edit: Bit shift is a simplified way, and actually should be 0xFF x (0x3F / 0xFF) to convert from 0～255 to 0～63.