Describe flash scanline effect

Based on pokeemerald's functions of the same name.

1 files changed, 75 insertions, 0 deletions

diff --git a/src/field_screen_effect.c b/src/field_screen_effect.c
new file mode 100644
index 000000000..19ed25950
--- /dev/null
+++ b/src/field_screen_effect.c
@@ -0,0 +1,75 @@
+#include "global.h"
+
+void SetFlashScanlineEffectWindowBoundary(u16 *dest, u32 y, s32 left, s32 right)
+{
+    if (y <= 160)
+    {
+        if (left < 0)
+            left = 0;
+        if (left > 255)
+            left = 255;
+        if (right < 0)
+            right = 0;
+        if (right > 255)
+            right = 255;
+        dest[y] = (left << 8) | right;
+    }
+}
+
+/*
+ * Draws a circle by approximating xy² + yx² = radius².
+ *
+ * error is approximately xy² - yx². Negative values mean the circle is
+ * slightly too large, and positive values mean the circle is slightly
+ * too small. By decreasing xy whenever the error becomes negative the
+ * code slightly under-approximates the size of the circle.
+ *
+ * The subtractive terms compute yx² - (yx - 1)², and therefore the sum
+ * is yx² - 1:
+ *   yx               |  0 |  1 |  2 |  3 |  4 |  5 |  6 |  7
+ *   (yx * 2) - 1     | -1 |  1 |  3 |  5 |  7 |  9 | 11 | 13
+ *   yx² - (yx - 1)²  | -1 |  1 |  3 |  5 |  7 |  9 | 11 | 13
+ *   cumulative error | -1 |  0 |  3 |  8 | 15 | 24 | 35 | 48
+ *   yx²              |  0 |  1 |  4 |  9 | 16 | 25 | 36 | 49
+ *
+ * The additive terms compute xy² - (xy - 1)² - 1, and therefore the sum
+ * (badly) approximates Σi² - (i - 1)², i ∈ (xy, r), consider r = 18:
+ *   xy               | 18 | 17 | 16 |  15 |  14 |  13 | ... |   0
+ *   xy² - (xy - 1)²  | 35 | 33 | 31 |  29 |  27 |  25 | ... |  -1
+ *   2 * (xy - 1)     | 34 | 32 | 30 |  28 |  26 |  24 | ... |  -2
+ *   cumulative error | 34 | 66 | 96 | 124 | 150 | 174 | ... | 304
+ *   Σi² - (i - 1)²   | 35 | 68 | 99 | 128 | 155 | 180 | ... | 323
+ * 18² = 324, so if the iterations ran until xy = 0 the cumulative error
+ * would be xy² - r.
+ *
+ * The error is initialized to r, which corrects for the error in the
+ * additive terms. In practice all r iterations don't occur because we
+ * early-exit when yx > xy, so it's half-way between a fix for that
+ * error and an approximation of the midpoint between r² and (r + 1)².
+ *
+ * The algorithm takes advantage of symmetry to compute boundaries in
+ * both directions out from centerY (using yx for y), and also both
+ * directions *in* from centerY ± radius (using xy for y). Because xy
+ * doesn't change on every iteration, we will frequently overwrite
+ * boundaries set in the previous iteration.
+ */
+void SetFlashScanlineEffectWindowBoundaries(u16 *dest, s32 centerX, s32 centerY, s32 radius)
+{
+    s32 xy = radius;
+    s32 error = radius;
+    s32 yx = 0;
+    while (xy >= yx)
+    {
+        SetFlashScanlineEffectWindowBoundary(dest, centerY - yx, centerX - xy, centerX + xy);
+        SetFlashScanlineEffectWindowBoundary(dest, centerY + yx, centerX - xy, centerX + xy);
+        SetFlashScanlineEffectWindowBoundary(dest, centerY - xy, centerX - yx, centerX + yx);
+        SetFlashScanlineEffectWindowBoundary(dest, centerY + xy, centerX - yx, centerX + yx);
+        error -= (yx * 2) - 1;
+        yx++;
+        if (error < 0)
+        {
+            error += 2 * (xy - 1);
+            xy--;
+        }
+    }
+}