Fix: Resolve BG color brightness regression, persist Time System tint, and refactor Minecart data
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scawful
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Docs/Issues/BGColor_Overlay_RootCause.md
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Docs/Issues/BGColor_Overlay_RootCause.md
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# BG Color / Overlay Regression — Resolved (March 2026)
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## Resolution Summary
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The "Too Bright" and "Flash to Day" bugs have been resolved by fixing the register management in `ZSCustomOverworld.asm`.
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**The Problem:**
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When transitioning from an area with an overlay (like Rain/Storms) to an area without one (Overlay ID `$FF`), the code cleared the Subscreen Enable register (`$1D`) but **failed to clear the Color Math Control register (`$9A`)**.
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- Rain sets `$9A` to `$72` (Additive Math).
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- If `$9A` remains `$72` in a normal area, the SNES PPU continues to perform additive color math using the Fixed Color registers (`$9C`/`$9D`).
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- This caused the background to appear significantly brighter (approx +6 per channel), turning the dark night tint into a "bright yellow-ish green".
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- This also caused the "Flash to Day" effect during transitions, as the additive brightness kicked in immediately.
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**The Fix:**
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Modified `ZSCustomOverworld.asm` in two key locations to ensure `$9A` is always cleared when no overlay is present.
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1. **Walking Transitions (`Overworld_ReloadSubscreenOverlay_Interupt`):**
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Added a check for Overlay `$FF` to explicitly clear `$9A`. This prevents the brightness glitch during scrolling.
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```asm
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; In Overworld_ReloadSubscreenOverlay_Interupt
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CPX.b #$FF : BNE .checkScroll
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LDA.b #$00 ; Disable Color Math
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BRA .loadOverlay
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```
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2. **Dungeon/Warp/Bird Transitions (`Overworld_LoadBGColorAndSubscreenOverlay`):**
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Added `STZ.b $9A` to the block handling the `$FF` case. This prevents the glitch when exiting dungeons or warping.
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```asm
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; In Overworld_LoadBGColorAndSubscreenOverlay
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CMP.w #$00FF : BNE .noCustomFixedColor
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SEP #$30
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STZ.b $9A ; FIX: Clear color math
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; ...
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```
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## Verification
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- **Brightness:** The background color in normal areas should now correctly reflect the Time System tint without extra brightness.
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- **Transitions:** Walking from a Rain area to a Normal area should no longer result in a brightness jump.
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- **Song of Storms:** Summoning and dismissing storms should work correctly, with the overlay and color math engaging and disengaging as expected.
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## Technical Details
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- **File:** `Overworld/ZSCustomOverworld.asm`
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- **Routines:** `Overworld_LoadBGColorAndSubscreenOverlay`, `Overworld_ReloadSubscreenOverlay_Interupt`.
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- **Registers:** `$1D` (Subscreen), `$9A` (CGADDSUB Mirror), `$9C`/`$9D` (COLDATA Mirrors).
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## Outstanding Issues
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- None related to BG Color Brightness.
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@@ -191,7 +191,7 @@ Palette_ArmorAndGloves:
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; =========================================================
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; Overworld Palette Persist
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Overworld_CgramAuxToMain:
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Oracle_CgramAuxToMain_Impl:
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{
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; Copies the auxiliary CGRAM buffer to the main one
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; Causes NMI to reupload the palette.
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@@ -213,14 +213,36 @@ Overworld_CgramAuxToMain:
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SEP #$20
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; Fix BG color: Apply time tinting after aux->main copy
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; Only when outdoors ($1B = 0)
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; NOTE: Overworld_LoadPalettes does NOT load palette 0 (BG color), so aux buffer
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; at offset 0 contains garbage. Must load from Pool_BGColorTable instead.
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LDA.b $1B : BNE .skip_bg_fix
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REP #$20
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; Get area index and load BG color from Pool_BGColorTable ($288000)
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LDA.b $8A : AND.w #$00FF : ASL : TAX
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LDA.l $288000, X ; Pool_BGColorTable
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BEQ .skip_bg_fix_16bit ; Skip if transparent ($0000)
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STA.l TimeState.SubColor
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JSL ColorSubEffect ; Apply time tinting, result in A
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; Write tinted color to all 4 BG color buffers
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STA.l $7EC500 ; PalCgram500_HUD
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STA.l $7EC300 ; PalBuf300_HUD
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STA.l $7EC540 ; PalCgram540_BG
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STA.l $7EC340 ; PalBuf340_BG
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.skip_bg_fix_16bit
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SEP #$20
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.skip_bg_fix
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; tell NMI to upload new CGRAM data
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INC $15
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RTL
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}
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pushpc
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org $02C769 ; Overworld_CgramAuxToMain
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JSL Overworld_CgramAuxToMain
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org $02C769 ; Hook vanilla Overworld_CopyPalettesToCache
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JSL Oracle_CgramAuxToMain_Impl
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RTS
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pullpc
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@@ -2364,6 +2364,13 @@ Overworld_ReloadSubscreenOverlay_Interupt:
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CPX.b #$95 : BEQ .loadOverlay ; Sky
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CPX.b #$9C : BEQ .loadOverlay ; Lava
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CPX.b #$96 : BEQ .loadOverlay ; Pyramid BG
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; Check for NO OVERLAY ($FF)
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CPX.b #$FF : BNE .checkScroll
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LDA.b #$00 ; Disable Color Math
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BRA .loadOverlay
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.checkScroll
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; TODO: Investigate what these checks are for.
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LDX.b $11 : CPX.b #$23 : BEQ .loadOverlay
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CPX.b #$2C : BEQ .loadOverlay
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@@ -3713,6 +3720,9 @@ Overworld_LoadBGColorAndSubscreenOverlay:
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SEP #$30 ; Set A, X, and Y in 8bit mode.
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; Clear color math control register to prevent brightness bug
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STZ.b $9A
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; Don't set the subscreen during a warp to hide the transparent
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; color change. This will get set properly later in the warp
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; but not everywhere else.
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@@ -305,28 +305,7 @@ Sprite_Minecart_Prep:
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PLB
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RTL
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; This is which room each track should start in if it hasn't already
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; been given a track.
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.TrackStartingRooms
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dw $0098, $0088, $0087, $0088, $0089, $0089, $0089, $0089
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dw $0089, $0089, $0089, $0089, $0089, $0089, $0089, $0089
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dw $0089, $0089, $0089, $0089, $0089, $0089, $0089, $0089
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dw $0089, $0089, $0089, $0089, $0089, $0089, $0089, $0089
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; This is where within the room each track should start in if it hasn't
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; already been given a position. This is necessary to allow for more
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; than one stopping point to be in one room.
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.TrackStartingX
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dw $1190, $1160, $1300, $1100, $1300, $1300, $1300, $1300
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dw $1300, $1300, $1300, $1300, $1300, $1300, $1300, $1300
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dw $1300, $1300, $1300, $1300, $1300, $1300, $1300, $1300
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dw $1300, $1300, $1300, $1300, $1300, $1300, $1300, $1300
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.TrackStartingY
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dw $1380, $10C9, $1100, $10D0, $1100, $1100, $1100, $1100
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dw $1100, $1100, $1100, $1100, $1100, $1100, $1100, $1100
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dw $1100, $1100, $1100, $1100, $1100, $1100, $1100, $1100
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dw $1100, $1100, $1100, $1100, $1100, $1100, $1100, $1100
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incsrc "data/minecart_tracks.asm"
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}
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; =========================================================
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