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#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--;
}
}
}
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