Palette fade

Monster data
HP	N/A
Points	N/A

Entity data
Entity pointer	$82:AB95

Palette fade is a boss monster. Though implemented in the same way as other boss monsters, this is not a monster in the literal sense.

Overview

A palette fade will cause the background and sprite palettes to transition from their starting values to the specified palettes over time at the start of a level. This is generally used to make the level appear to go from day to night.

Once the palette fade is completed, two additional effects are activated. Werewolves will be allowed to spawn, and tourists will transform into werewolves. If either of these effects is desired without having a visible palette fade, then the post-fade palettes can be set to the same values as the starting palettes. This will cause the fade to complete quickly, essentially activating werewolves at the beginning of the level.

Details

The process used to fade between the palettes is as follows. The palette fade starts at palette 0, and on each iteration will update all 16 colors in the background and sprite palettes at the current index. After each iteration, the current palette index is incremented. When all 8 palettes have been updated, the palette fade will complete if there were no colors updated during the last 8 iterations. If there were updated colors, then current palette is reset to 0 and the process repeats. Background palette 7 is skipped during this process, but sprite palette 7 is updated as normal.

Before each iteration, there is a delay. The delay starts at 64 frames, and this value is decremented between each iteration. The delay will never go below 1 frame.

To update a color, compare each of the red, green, and blue components of the current color and the target color. If the current value is less than the target value, increment it, if it is greater, decrement it, and if the values are the same, do not alter it. This process means that the number of iterations required to complete the fade depends on the largest difference between starting and target color components. See timing data for details.

Additional data format

The format for the additional data in the level is:

Offset	Length	Type	Name	Description
0x00	4	pointer32	targetBGPalette	Pointer to target background palette
0x04	4	pointer32	targetSpritePalette	Pointer to target sprite palette

RAM map

Entity arguments

Address	Length	Type	Name	Description
$00	4	pointer32	additionalData	Pointer to additional data

Entity memory

Address	Length	Type	Name	Description
$0A	2	boolean	updated	Any palette has been updated in current 8-palette cycle
$0C	2	uint16	curIndex	Index of current palette (in bytes)
$0E	2	uint16	curDelay	Current delay between iterations
$10	4	pointer32	targetBGPalette	Pointer to target background palette
$14	4	pointer32	targetSpritePalette	Pointer to target sprite palette

Bugs

If the high bit is set on any of the colors in the target palettes, then the palette fade will never be considered complete. This happens because the color update logic checks if the color values are exactly equal when determining if the fade is complete, but will never set this bit on the updated colors. Since the high bit is unused in SNES colors, this can be completely avoided by simply leaving the bit unset on all colors. None of the colors in the base game ever have this bit set, so this bug can only occur in modified ROMs.

Timing Data

Largest component difference	Duration (frames)	Duration (seconds)
0	484	8.053405392
1	904	15.04189768
2	1260	20.96547685
3	1552	25.82414291
4	1780	29.61789586
5	1944	32.34673571
6	2044	34.01066244
7	2080	34.60967607
8	2088	34.74279021
9	2096	34.87590434
10	2104	35.00901848
11	2112	35.14213262
12	2120	35.27524676
13	2128	35.4083609
14	2136	35.54147504
15	2144	35.67458918
16	2152	35.80770332
17	2160	35.94081745
18	2168	36.07393159
19	2176	36.20704573
20	2184	36.34015987
21	2192	36.47327401
22	2200	36.60638815
23	2208	36.73950229
24	2216	36.87261642
25	2224	37.00573056
26	2232	37.1388447
27	2240	37.27195884
28	2248	37.40507298
29	2256	37.53818712
30	2264	37.67130126
31	2272	37.8044154

Highlighted rows in the table below are palette combinations that occur in the original game.

Tileset	Palette 1	Palette 2	Largest component difference	Duration (seconds)
Grass	Autumn	Night	24	36.87261642
	Autumn	Normal	18	36.07393159
	Autumn	Winter	28	37.40507298
	Night	Normal	30	37.67130126
	Night	Winter	30	37.67130126
	Normal	Winter	23	36.73950229
Castle	Bright	Dark	24	36.87261642
	Bright	Night	24	36.87261642
	Bright	Normal	24	36.87261642
	Dark	Night	17	35.94081745
	Dark	Normal	14	35.54147504
	Night	Normal	21	36.47327401
Mall+Factory	Factory	Mall	27	37.27195884
Office+Cave	Dark Fire Cave	Dark	31	37.8044154
	Dark Fire Cave	Fire Cave	31	37.8044154
	Dark Fire Cave	Light	31	37.8044154
	Dark Fire Cave	Normal	31	37.8044154
	Dark	Fire Cave	31	37.8044154
	Dark	Light	24	36.87261642
	Dark	Normal	31	37.8044154
	Fire Cave	Light	31	37.8044154
	Fire Cave	Normal	18	36.07393159
	Light	Normal	31	37.8044154
Sand	Beach	Dark Beach	23	36.73950229
	Beach	Mines	22	36.60638815
	Beach	Pyramid	22	36.60638815
	Dark Beach	Mines	18	36.07393159
	Dark Beach	Pyramid	23	36.73950229
	Mines	Pyramid	22	36.60638815

Pseudocode

updateColor(targetColor, currentColor) {
	if (currentColor == targetColor) {
		return false
	}

	targetB = targetColor & 0x7C00
	targetG = targetColor & 0x03E0
	targetR = targetColor & 0x001F

	newB = currentColor & 0x7C00
	if (newB > targetB) {
		newB -= 0x400
	} else if (newB < targetB) {
		newB += 0x400
	}

	newG = currentColor & 0x03E0
	if (newG > targetG) {
		newG -= 0x20
	} else if (newG < targetG) {
		newG += 0x20
	}

	newR = currentColor & 0x001F
	if (newR > targetR) {
		newR -= 1
	} else if (newR < targetR) {
		newR += 1
	}
	
	newColor = newR | newG | newB
	return true, newColor
}

updateBGPalette(targetPalette, index) {
	if (index >= 0x70) {
		return false
	}

	colorUpdated = false
	for (i = index; i < index + 0x10; i += 1) {
		updated, newColor = updateColor(targetPalette[i], currentBGPalette[i])
		if (updated) {
			colorUpdated = true
			currentBGPalette[i] = newColor
			animatedBGPalette[i] = newColor
		}
	}
	return colorUpdated
}

updateSpritePalette(targetPalette, index) {
	colorUpdated = false
	for (i = index; i < index + 0x10; i += 1) {
		updated, newColor = updateColor(targetPalette[i], currentSpritePalette[i])
		if (updated) {
			colorUpdated = true
			currentSpritePalette[i] = newColor
		}
	}
	return colorUpdated
}

main() {
	this.targetBGPalette = args.additionalData.targetBGPalette
	this.targetSpritePalette = args.additionalData.targetSpritePalette
	this.curIndex = 0
	this.updated = false
	this.curDelay = 64

	while (true) {
		waitFrames(this.curDelay)
		if (updateBGPalette(this.targetBGPalette, this.curIndex)) {
			this.updated = true
		}
		if (updateSpritePalette(this.targetSpritePalette, this.curIndex)) {
			this.updated = true
		}

		this.curIndex += 0x10
		if (this.curIndex >= 0x80) {
			if (!this.updated) {
				paletteFadeComplete = true
				return
			}
			runOnVBLANK($80:9FDF) // Transfer palette to CG-RAM
			this.updated = false
			this.curIndex = 0
			if (this.curDelay >= 2) {
				this.curDelay -= 1
			}
		}
	}
}