Barycentric Rasterization

Achieving rasterization with BaryCentric Coordinate.

1. Introduction

2. Implementation

We are implementing barycentric-based rasterization using Vulkan compute shader.
Code is available at: https://github.com/chuzcjoe/CORE/tree/master/examples/vulkan/ComputeBarycentricRasterizer

The core part is the shader implementation. This is identical to the math above.

#version 450

layout(binding = 0) uniform UniformBufferObject {
    int width;
    int height;
    vec2 vertex_a;
    vec2 vertex_b;
    vec2 vertex_c;
}
ubo;

layout(std430, binding = 1) buffer rasterize_in_out { int rasterize[]; };

layout(local_size_x = 16, local_size_y = 16, local_size_z = 1) in;

int CalArea(ivec2 p1, ivec2 p2, ivec2 p3) {
    return (p2.x - p1.x) * (p3.y - p1.y) - (p2.y - p1.y) * (p3.x - p1.x);
}

bool BaryCentric(ivec2 pos) {
    ivec2 p1 = ivec2(ubo.vertex_a);
    ivec2 p2 = ivec2(ubo.vertex_b);
    ivec2 p3 = ivec2(ubo.vertex_c);

    int area1 =  CalArea(p1, p2, pos);
    int area2 = CalArea(p2, p3, pos);
    int area3 = CalArea(p3, p1, pos);

    return area1 >= 0 && area2 >= 0 && area3 >= 0;
}

void main() {
    ivec2 gid = ivec2(gl_GlobalInvocationID.xy);
    const int row = gid.y;
    const int col = gid.x;
    if (row >= ubo.height || col >= ubo.width) return;

    int ax = ivec2(ubo.vertex_a).x;
    int ay = ivec2(ubo.vertex_a).y;
    int bx = ivec2(ubo.vertex_b).x;
    int by = ivec2(ubo.vertex_b).y;
    int cx = ivec2(ubo.vertex_c).x;
    int cy = ivec2(ubo.vertex_c).y;

    int top = min(min(ay, by), cy);
    int bottom = max(max(ay, by), cy);
    int left = min(min(ax, bx), cx);
    int right = max(max(ax, bx), cx);

    if (row < top || row > bottom || col < left || col > right) return;

    // rasterize
    bool in_triangle = BaryCentric(ivec2(col, row));
    if (in_triangle) {
        rasterize[row * ubo.width + col] = 1;
    }
}

We write white color(255) to the rasterized pixels and the rest remain black.