Fading colors

=Palette fade=

Real-time
Palettes can be faded to white or black by respectively incrementing or decrementing each color component of each color individually. Of course, care has to be taken so that each value stops at 31 or 0.

Fading to a specific color can also be done this way, by comparing each color components with the target ones and incrementing/decrementing them accordingly (see example code).

Pre-computed
Real time fading is very flexible but also very CPU intensive. Another way to fade palettes is to pre-compute all the steps and store them into ROM for simple lookup. This method requires very little CPU but uses more ROM space.

=Simulated fade=





Since it has the highest priority on the screen, the fix layer can be used as a mask to fade the underlaying display by using dithered tiles. This can only be done if the fix doesn't have to show important information during fading.

Metal Slug 3 clears the HUD then does this for wiping transitions between level sections.

=Real-time fading code=

RGB components can then be compared with a target palette in RAM and adjusted until they match, for example:

Note that with such a constant increment/decrement value, some components will reach their target before others. Doing a correct proportional fade would involve fixed point math, which probably isn't worth the trouble.