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Fix DrawLineAa anti-aliasing inconsistency at certain angles #95

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Fix DrawLineAa anti-aliasing inconsistency at certain angles #95

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a72fcf9
Initial plan for issue
Copilot May 21, 2025
6cf171f
Fix anti-aliasing in DrawLineAa method
Copilot May 21, 2025
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276 changes: 143 additions & 133 deletions Source/WriteableBitmapEx/WriteableBitmapAntialiasingExtensions.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -318,139 +318,149 @@ private static void Swap<T>(ref T a, ref T b)
b = t;
}

private static void AALineQ1(int width, int height, BitmapContext context, int x1, int y1, int x2, int y2, Int32 color, bool minEdge, bool leftEdge)
{
Byte off = 0;

if (minEdge) off = 0xff;

if (x1 == x2) return;
if (y1 == y2) return;

var buffer = context.Pixels;

if (y1 > y2)
{
Swap(ref x1, ref x2);
Swap(ref y1, ref y2);
}

int deltax = (x2 - x1);
int deltay = (y2 - y1);

if (x1 > x2) deltax = (x1 - x2);

int x = x1;
int y = y1;

UInt16 m = 0;

if (deltax > deltay) m = (ushort)(((deltay << 16) / deltax));
else m = (ushort)(((deltax << 16) / deltay));

UInt16 e = 0;

var a = (byte)((color & 0xff000000) >> 24);
var r = (byte)((color & 0x00ff0000) >> 16);
var g = (byte)((color & 0x0000ff00) >> 8);
var b = (byte)((color & 0x000000ff) >> 0);

Byte rs, gs, bs;
Byte rd, gd, bd;

Int32 d;

Byte ta = a;

e = 0;

if (deltax >= deltay)
{
while (deltax-- != 0)
{
if ((UInt16)(e + m) <= e) // Roll
{
y++;
}

e += m;

if (x1 < x2) x++;
else x--;

if (y < 0 || y >= height) continue;

if (leftEdge) leftEdgeX[y] = Math.Max(x + 1, leftEdgeX[y]);
else rightEdgeX[y] = Math.Min(x - 1, rightEdgeX[y]);

if (x < 0 || x >= width) continue;

//

ta = (byte)((a * (UInt16)(((((UInt16)(e >> 8))) ^ off))) >> 8);

rs = r;
gs = g;
bs = b;

d = buffer[y * width + x];

rd = (byte)((d & 0x00ff0000) >> 16);
gd = (byte)((d & 0x0000ff00) >> 8);
bd = (byte)((d & 0x000000ff) >> 0);

rd = (byte)((rs * ta + rd * (0xff - ta)) >> 8);
gd = (byte)((gs * ta + gd * (0xff - ta)) >> 8);
bd = (byte)((bs * ta + bd * (0xff - ta)) >> 8);

buffer[y * width + x] = (0xff << 24) | (rd << 16) | (gd << 8) | (bd << 0);

//
}
}
else
{
off ^= 0xff;

while (--deltay != 0)
{
if ((UInt16)(e + m) <= e) // Roll
{
if (x1 < x2) x++;
else x--;
}

e += m;

y++;

if (x < 0 || x >= width) continue;
if (y < 0 || y >= height) continue;

//

ta = (byte)((a * (UInt16)(((((UInt16)(e >> 8))) ^ off))) >> 8);

rs = r;
gs = g;
bs = b;

d = buffer[y * width + x];

rd = (byte)((d & 0x00ff0000) >> 16);
gd = (byte)((d & 0x0000ff00) >> 8);
bd = (byte)((d & 0x000000ff) >> 0);

rd = (byte)((rs * ta + rd * (0xff - ta)) >> 8);
gd = (byte)((gs * ta + gd * (0xff - ta)) >> 8);
bd = (byte)((bs * ta + bd * (0xff - ta)) >> 8);

buffer[y * width + x] = (0xff << 24) | (rd << 16) | (gd << 8) | (bd << 0);

if (leftEdge) leftEdgeX[y] = x + 1;
else rightEdgeX[y] = x - 1;
}
}
private static void AALineQ1(int width, int height, BitmapContext context, int x1, int y1, int x2, int y2, Int32 color, bool minEdge, bool leftEdge)
{
Byte off = 0;

if (minEdge) off = 0xff;

if (x1 == x2) return;
if (y1 == y2) return;

var buffer = context.Pixels;

if (y1 > y2)
{
Swap(ref x1, ref x2);
Swap(ref y1, ref y2);
}

int deltax = (x2 - x1);
int deltay = (y2 - y1);

if (x1 > x2) deltax = (x1 - x2);

int x = x1;
int y = y1;

UInt16 m = 0;

if (deltax > deltay) m = (ushort)(((deltay << 16) / deltax));
else m = (ushort)(((deltax << 16) / deltay));

UInt16 e = 0;

var a = (byte)((color & 0xff000000) >> 24);
var r = (byte)((color & 0x00ff0000) >> 16);
var g = (byte)((color & 0x0000ff00) >> 8);
var b = (byte)((color & 0x000000ff) >> 0);

Byte rs, gs, bs;
Byte rd, gd, bd;

Int32 d;

Byte ta = a;

e = 0;

if (deltax >= deltay)
{
while (deltax-- != 0)
{
if ((UInt16)(e + m) <= e) // Roll
{
y++;
}

e += m;

if (x1 < x2) x++;
else x--;

if (y < 0 || y >= height) continue;

if (leftEdge) leftEdgeX[y] = Math.Max(x + 1, leftEdgeX[y]);
else rightEdgeX[y] = Math.Min(x - 1, rightEdgeX[y]);

if (x < 0 || x >= width) continue;

//

// Calculate transparency based on error term for anti-aliasing
UInt16 errorValue = (UInt16)(e >> 8);
if (off != 0)
errorValue = (UInt16)(0xFF - errorValue);
ta = (byte)((a * errorValue) >> 8);

rs = r;
gs = g;
bs = b;

d = buffer[y * width + x];

rd = (byte)((d & 0x00ff0000) >> 16);
gd = (byte)((d & 0x0000ff00) >> 8);
bd = (byte)((d & 0x000000ff) >> 0);

rd = (byte)((rs * ta + rd * (0xff - ta)) >> 8);
gd = (byte)((gs * ta + gd * (0xff - ta)) >> 8);
bd = (byte)((bs * ta + bd * (0xff - ta)) >> 8);

buffer[y * width + x] = (0xff << 24) | (rd << 16) | (gd << 8) | (bd << 0);

//
}
}
else
{
// No need to invert off for vertical lines
// This ensures consistent anti-aliasing for all line angles

while (--deltay != 0)
{
if ((UInt16)(e + m) <= e) // Roll
{
if (x1 < x2) x++;
else x--;
}

e += m;

y++;

if (x < 0 || x >= width) continue;
if (y < 0 || y >= height) continue;

//

// Calculate transparency based on error term for anti-aliasing
// Using same calculation as horizontal case for consistency
UInt16 errorValue = (UInt16)(e >> 8);
if (off != 0)
errorValue = (UInt16)(0xFF - errorValue);
ta = (byte)((a * errorValue) >> 8);

rs = r;
gs = g;
bs = b;

d = buffer[y * width + x];

rd = (byte)((d & 0x00ff0000) >> 16);
gd = (byte)((d & 0x0000ff00) >> 8);
bd = (byte)((d & 0x000000ff) >> 0);

rd = (byte)((rs * ta + rd * (0xff - ta)) >> 8);
gd = (byte)((gs * ta + gd * (0xff - ta)) >> 8);
bd = (byte)((bs * ta + bd * (0xff - ta)) >> 8);

buffer[y * width + x] = (0xff << 24) | (rd << 16) | (gd << 8) | (bd << 0);

if (leftEdge) leftEdgeX[y] = x + 1;
else rightEdgeX[y] = x - 1;
}
}
}
}
}
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