From 090c4f39362d2852ff3071e012425a8ab569b803 Mon Sep 17 00:00:00 2001
From: Ariel A <24759293+aarant@users.noreply.github.com>
Date: Fri, 24 Jan 2025 02:02:21 -0500
Subject: [PATCH] feat: hardware fades now affect BLDALPHA too

---
 include/gba/io_reg.h |  3 +++
 src/palette.c        | 33 +++++++++++++++++++++++++++++++++
 2 files changed, 36 insertions(+)

diff --git a/include/gba/io_reg.h b/include/gba/io_reg.h
index 53021185e3..75e3b56117 100644
--- a/include/gba/io_reg.h
+++ b/include/gba/io_reg.h
@@ -599,6 +599,7 @@
 #define BLDCNT_EFFECT_BLEND     (1 << 6)   // 1st+2nd targets mixed (controlled by BLDALPHA)
 #define BLDCNT_EFFECT_LIGHTEN   (2 << 6)   // 1st target becomes whiter (controlled by BLDY)
 #define BLDCNT_EFFECT_DARKEN    (3 << 6)   // 1st target becomes blacker (controlled by BLDY)
+#define BLDCNT_EFFECT_EFF_MASK  (3 << 6)    // mask to check effect
 // Bits 8-13 select layers for the 2nd target
 #define BLDCNT_TGT2_BG0      (1 << 8)
 #define BLDCNT_TGT2_BG1      (1 << 9)
@@ -611,6 +612,8 @@
 
 // BLDALPHA
 #define BLDALPHA_BLEND(target1, target2) (((target2) << 8) | (target1))
+#define BLDALPHA_TGT1(bld) ((bld) & 0x1F)
+#define BLDALPHA_TGT2(bld) (((bld) >> 8) & 0x1F)
 
 // SOUNDCNT_H
 #define SOUND_CGB_MIX_QUARTER 0x0000
diff --git a/src/palette.c b/src/palette.c
index 3ea2a08195..303e6ced42 100644
--- a/src/palette.c
+++ b/src/palette.c
@@ -962,10 +962,43 @@ static u8 UpdateHardwarePaletteFade(void)
     return gPaletteFade.active ? PALETTE_FADE_STATUS_ACTIVE : PALETTE_FADE_STATUS_DONE;
 }
 
+// Only called for hardware fades
 static void UpdateBlendRegisters(void)
 {
     SetGpuReg(REG_OFFSET_BLDCNT, (u16)gPaletteFade_blendCnt);
     SetGpuReg(REG_OFFSET_BLDY, gPaletteFade.y);
+    // If fade-out, also adjust BLDALPHA and DISPCNT
+    if (!gPaletteFade.yDec /*&& gPaletteFade.mode == HARDWARE_FADE*/) {
+        u16 bldAlpha = GetGpuReg(REG_OFFSET_BLDALPHA);
+        u8 tgt1 = BLDALPHA_TGT1(bldAlpha);
+        u8 tgt2 = BLDALPHA_TGT2(bldAlpha);
+        u8 bldFade;
+
+        switch (gPaletteFade_blendCnt & BLDCNT_EFFECT_EFF_MASK)
+        {
+        // FADE_TO_BLACK
+        case BLDCNT_EFFECT_DARKEN:
+            bldFade = BLDALPHA_TGT1(max(0, 16 - gPaletteFade.y));
+            SetGpuReg(
+                REG_OFFSET_BLDALPHA,
+                BLDALPHA_BLEND(min(tgt1, bldFade), min(tgt2, bldFade))
+            );
+            break;
+        // FADE_TO_WHITE
+        case BLDCNT_EFFECT_LIGHTEN:
+            SetGpuReg(
+                REG_OFFSET_BLDALPHA,
+                BLDALPHA_BLEND(min(++tgt1, 31), min(++tgt2, 31))
+            );
+            // cause display to show white when finished
+            // (otherwise blend-mode sprites will still be visible)
+            if (gPaletteFade.hardwareFadeFinishing && gPaletteFade.y >= 16)
+                SetGpuRegBits(REG_OFFSET_DISPCNT, DISPCNT_FORCED_BLANK);
+            break;
+        }
+    } else
+        ClearGpuRegBits(REG_OFFSET_DISPCNT, DISPCNT_FORCED_BLANK);
+
     if (gPaletteFade.hardwareFadeFinishing)
     {
         gPaletteFade.hardwareFadeFinishing = FALSE;