Reduce hardcoded constants in Lights_GlowCheck, some more comments on the CPU driven depth test and z-buffer value conversion (#1506)

2 files changed, 18 insertions, 10 deletions

diff --git a/src/code/z_kankyo.c b/src/code/z_kankyo.c
index 84b4558e7..e560f7eb5 100644
--- a/src/code/z_kankyo.c
+++ b/src/code/z_kankyo.c
@@ -12,13 +12,14 @@ typedef enum {
 } LightningBoltState;
 
 typedef struct {
-    /* 0x00 */ s32 mantissa;
-    /* 0x04 */ s32 exponent;
-} ZBufValConversionEntry; // size = 0x8
+    /* 0x00 */ s32 mantissaShift; // shift applied to the mantissa of the z buffer value
+    /* 0x04 */ s32 base;          // 15.3 fixed-point base value for the exponent
+} ZBufValConversionEntry;         // size = 0x8
 
+// This table needs as many values as there are values for the 3-bit exponent
 ZBufValConversionEntry sZBufValConversionTable[1 << 3] = {
-    { 6, 0x00000 }, { 5, 0x20000 }, { 4, 0x30000 }, { 3, 0x38000 },
-    { 2, 0x3C000 }, { 1, 0x3E000 }, { 0, 0x3F000 }, { 0, 0x3F800 },
+    { 6, 0x0000 << 3 }, { 5, 0x4000 << 3 }, { 4, 0x6000 << 3 }, { 3, 0x7000 << 3 },
+    { 2, 0x7800 << 3 }, { 1, 0x7C00 << 3 }, { 0, 0x7E00 << 3 }, { 0, 0x7F00 << 3 },
 };
 
 u8 gWeatherMode = WEATHER_MODE_CLEAR; // "E_wether_flg"
@@ -225,9 +226,9 @@ u16 sSandstormScroll;
  *   4: dz value (unused)
  */
 s32 Environment_ZBufValToFixedPoint(s32 zBufferVal) {
-    // base[exp] + mantissa << shift[exp]
-    s32 ret = (ZBUFVAL_MANTISSA(zBufferVal) << sZBufValConversionTable[ZBUFVAL_EXPONENT(zBufferVal)].mantissa) +
-              sZBufValConversionTable[ZBUFVAL_EXPONENT(zBufferVal)].exponent;
+    // base[exp] + (mantissa << shift[exp])
+    s32 ret = (ZBUFVAL_MANTISSA(zBufferVal) << sZBufValConversionTable[ZBUFVAL_EXPONENT(zBufferVal)].mantissaShift) +
+              sZBufValConversionTable[ZBUFVAL_EXPONENT(zBufferVal)].base;
 
     return ret;
 }
diff --git a/src/code/z_lights.c b/src/code/z_lights.c
index b230100bc..f01c49df2 100644
--- a/src/code/z_lights.c
+++ b/src/code/z_lights.c
@@ -339,11 +339,18 @@ void Lights_GlowCheck(PlayState* play) {
             wY = multDest.y * cappedInvWDest;
 
             if ((multDest.z > 1.0f) && (fabsf(wX) < 1.0f) && (fabsf(wY) < 1.0f)) {
-                wZ = (s32)((multDest.z * cappedInvWDest) * 16352.0f) + 16352;
-                zBuf = gZBuffer[(s32)((wY * -120.0f) + 120.0f)][(s32)((wX * 160.0f) + 160.0f)] * 4;
+                // Compute screen z value assuming the viewport scale and translation both have value G_MAXZ / 2
+                // The multiplication by 32 follows from how the RSP microcode computes the screen z value.
+                wZ = (s32)((multDest.z * cappedInvWDest) * ((G_MAXZ / 2) * 32)) + ((G_MAXZ / 2) * 32);
+                // Obtain the z-buffer value for the screen pixel corresponding to the center of the glow.
+                zBuf = gZBuffer[(s32)((wY * -(SCREEN_HEIGHT / 2)) + (SCREEN_HEIGHT / 2))]
+                               [(s32)((wX * (SCREEN_WIDTH / 2)) + (SCREEN_WIDTH / 2))]
+                       << 2;
                 if (1) {}
                 if (1) {}
 
+                // Compare the computed screen z value to the integer part of the z-buffer value in fixed point. If
+                // it is less than the value from the z-buffer the depth test passes and the glow can draw.
                 if (wZ < (Environment_ZBufValToFixedPoint(zBuf) >> 3)) {
                     params->drawGlow = true;
                 }