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

File:Sengoku3fade.png

Fix fade to white in Sengoku 3's intro

Fix wipe in Metal Slug 3

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

; D0.w: NG palette word -> D2.b, D3.b, D4.b: RGB555 (ignores common bit)
NG2RGB:
    ;Red
    move.w  d0,d2                ;CrgbRRRRGGGGBBBB
    lsr.w   #8,d2                ;________CrgbRRRR
    andi.b  #$0F,d2              ;____________RRRR
    move.w  d0,d1                ;CrgbRRRRGGGGBBBB
    lsl.w   #2,d1                ;gbRRRRGGGGBBBBCr
    asl.b   #1,d2                ;___________RRRRr

    ;Green
    move.b  d0,d3                ;________GGGGBBBB
    lsr.b   #4,d3                ;____________GGGG
    lsl.w   #1,d1                ;bRRRRGGGGBBBBCrg
    asl.b   #1,d3                ;___________GGGGg

    ;Blue
    move.b  d0,d4                ;________GGGGBBBB
    andi.b  #$0F,d4              ;____________BBBB
    lsl.w   #1,d1                ;RRRRGGGGBBBBCrgb
    asl.b   #1,d4                ;___________BBBBb
    rts

; D2.b, D3.b, D4.b: RGB555 -> D0.w: NG palette word (ignores common bit)
RGB2NG:
    moveq.l #0,d0
    lsr.b   #1,d2                ;____________RRRR
    asl.b   #1,d0                ;_______________r
    lsr.b   #1,d3                ;____________GGGG
    asl.b   #1,d0                ;______________rg
    lsr.b   #1,d4                ;____________BBBB
    asl.b   #1,d0                ;_____________rgb

    lsl.b   #4,d0                ;_________rgb____
    or.b    d2,d0                ;_________rgbRRRR
    lsl.w   #4,d0                ;_____rgbRRRR____
    or.b    d3,d0                ;_____rgbRRRRGGGG
    lsl.w   #4,d0                ;_rgbRRRRGGGG____
    or.b    d4,d0                ;_rgbRRRRGGGGBBBB
    rts

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

; Compares red in D0 with D2, inc/dec depending on result
    cmp.b   d2,d0
    blo     .lower
    bhi     .higher
    bra     .same
.lower:
    addq.b  #1,d0
    bra     .same
.higher:
    subq.b  #1,d0
.same:

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.