barycentric.c

void RasterizeTriangleTextured(
    WindowData* pDat,
    Vector3 pts[3],      // screen.x,y in pixels; .z = NDC.z [-1..+1]
    float invWs[3],      // 1/clip.w at each vertex
    Vector2 uvs[3],      // UV coords [0..1]
    BYTE* texture,       // raw 24-bit BMP data (BGR, row aligned)
    int texWidth,
    int texHeight)
{
    // 1) Compute bounding box in integer pixels
    int minX = (int)fmaxf(0.0f, floorf(fminf(fminf(pts[0].x, pts[1].x), pts[2].x)));
    int maxX = (int)fminf((float)(pDat->width - 1), ceilf(fmaxf(fmaxf(pts[0].x, pts[1].x), pts[2].x)));
    int minY = (int)fmaxf(0.0f, floorf(fminf(fminf(pts[0].y, pts[1].y), pts[2].y)));
    int maxY = (int)fminf((float)(pDat->height - 1), ceilf(fmaxf(fmaxf(pts[0].y, pts[1].y), pts[2].y)));

    Vector2 a = { pts[0].x, pts[0].y };
    Vector2 b = { pts[1].x, pts[1].y };
    Vector2 c = { pts[2].x, pts[2].y };

    // Early exit if degenerate triangle (optional)
    Barycentric dummy = ComputeBarycentricsSIMD(a, b, c, a); // just to compute denom
    if (fabsf(dummy.lambda1 + dummy.lambda2 + dummy.lambda3 - 1.0f) > 1.01f) {
        // Could alternatively check denom inside ComputeBarycentrics
        return;
    }

    // 3) Loop over every pixel in bounding box
    for (int y = minY; y <= maxY; ++y) {
        for (int x = minX; x <= maxX; ++x) {
            Vector2 p = { (float)x + 0.5f, (float)y + 0.5f }; // sample center of pixel

            // 3.a) Compute barycentric coordinates using your helper
            Barycentric bary = ComputeBarycentricsSIMD(p, a, b, c);

            // 3.b) Cull pixels outside triangle
            if (bary.lambda1 < 0.0f || bary.lambda2 < 0.0f || bary.lambda3 < 0.0f) {
                continue;
            }

            // 4) Reconstruct NDC.z at pixel
            float ndcZ = bary.lambda1 * pts[0].z +
                bary.lambda2 * pts[1].z +
                bary.lambda3 * pts[2].z;

            int index = y * pDat->width + x;

            // 5) Depth test
            if (ndcZ >= pDat->zBuffer[index]) {
                continue;
            }

            // 6 & 7) Interpolate perspective-correct UV using your helper
            Vector2 uv = InterpolateUV(uvs, invWs, &bary);

            // 8) Clamp UV [0..1]
            uv.x = fmaxf(0.0f, fminf(1.0f, uv.x));
            uv.y = fmaxf(0.0f, fminf(1.0f, uv.y));

            // 9) Convert UV to texel indices
            int texX = (int)(uv.x * (texWidth - 1));
            int texY = (int)((1.0f - uv.y) * (texHeight - 1)); // flip v

            // 10) Fetch texel from BMP texture (4-byte aligned rows)
            int rowBytes = ((texWidth * 3 + 3) & ~3);
            BYTE* pixel = &texture[texY * rowBytes + texX * 3];

            // 11) Write pixel & update zBuffer
            pDat->zBuffer[index] = ndcZ;

            unsigned char* dst = (unsigned char*)pDat->pPixels + index * 4;
            dst[0] = pixel[0];   // B
            dst[1] = pixel[1];   // G
            dst[2] = pixel[2];   // R
            dst[3] = 255;        // A
        }
    }
}