When implementing subpixel precision in a software rasterizer I've found the following code:
It's just too bad the author doesn't explain anything to it. Can someone explain to me how shifting these integer variables gives us 4 bit subpixel precision a.k.a 16 more values of precision ? Not to mention this not working in my code :/
// 28.4 fixed-point coordinates
const int Y1 = iround(16.0f * v1.y);
const int Y2 = iround(16.0f * v2.y);
const int Y3 = iround(16.0f * v3.y);
const int X1 = iround(16.0f * v1.x);
const int X2 = iround(16.0f * v2.x);
const int X3 = iround(16.0f * v3.x);
// Fixed-point deltas
const int FDX12 = DX12 << 4;
const int FDX23 = DX23 << 4;
const int FDX31 = DX31 << 4;
const int FDY12 = DY12 << 4;
const int FDY23 = DY23 << 4;
const int FDY31 = DY31 << 4;
// Bounding rectangle
int minx = (min(X1, X2, X3) + 0xF) >> 4;
int maxx = (max(X1, X2, X3) + 0xF) >> 4;
int miny = (min(Y1, Y2, Y3) + 0xF) >> 4;
int maxy = (max(Y1, Y2, Y3) + 0xF) >> 4;
int CY1 = C1 + DX12 * (miny << 4) - DY12 * (minx << 4);
int CY2 = C2 + DX23 * (miny << 4) - DY23 * (minx << 4);
int CY3 = C3 + DX31 * (miny << 4) - DY31 * (minx << 4);
for(int y = miny; y < maxy; y++)
{
int CX1 = CY1;
int CX2 = CY2;
int CX3 = CY3;
for(int x = minx; x < maxx; x++)
{
if(CX1 > 0 && CX2 > 0 && CX3 > 0)
{
colorBuffer[x] = 0x00FFFFFF;
}
CX1 -= FDY12;
CX2 -= FDY23;
CX3 -= FDY31;
}
CY1 += FDX12;
CY2 += FDX23;
CY3 += FDX31;
}
) integer with the value 3
