Trouble With Barycentric Rasterizer (based on the ryg blog) - c

Question

I have been trying to implement the optimized rasterizer outlined in this blog: https://fgiesen.wordpress.com/2013/02/10/optimizing-the-basic-rasterizer/.
The naive approach outlined in his prior blog post https://fgiesen.wordpress.com/2013/02/08/triangle-rasterization-in-practice/, calculates the determinants (for barycentric weights) at each pixel. But his optimized version takes advantage of the fact that for three points a, b, c, the determinant function
(b.x - a.x) * (c.y - a.y) - (b.y - a.y) * (c.x - a.x)
can be rewritten as
A * c.x + B * c.y + C
for
A = (a.y - b.y), B = (b.x - a.x), C = a.x * b.y - a.y * b.x.
Since during traversal, the points a, b are two points on the triangle, c is the only point whose values change.
For any edge function E which outputs the weight for the corresponding triangle partition made by it and the point c, we can express an x difference and y difference in discrete steps.
E(c.x, c.y) = A * c.x + B * c.y + C
so
E(c.x + 1, c.y) - E(c.x, c.y) = A, and E(c.x, c.y + 1) - E(c.x, c.y) = B
So at each iteration instead of recalculating the determinant, we can just find the three determinants for the first c, and then increment them the A or B which corresponds to their edge.
I'm currently trying to implement this in my own rasterizer, but I quickly noticed an issue with the formula. My triangles are fed to the draw function in screen space, so a low y value means high up whereas in vector space it means low down. I thought I would account for this by multiplying every y value in the formula by -1.
This gave me the following formula for the determinant of a, b, c:
(b.x - a.x) * (a.y - c.y) - (a.y - b.y) * (c.x - a.x)
From which I derived the following A, B, C:
A = b.y - a.y, B = a.x - b.x, C = b.x * a.y - a.x * b.y
In my tests, using this new determinant formula (calculated at every pixel) works fine. And for the first point c in traversal, it is equivalent to
A * c.x + B * c.y + C
But as it continues traversing along the triangle's bounding box, the step incremented determinant values go out of sync with the raw calculated determinant values. Somehow this means that the step sizes A and B are faulty-- which makes no sense to me.
The only two causes of this problem I can think of are either I calculated A B and C incorrectly, or I am not mapping from vector space to screen space in a way that preserves area or orientation.
But just in case, here is all of my code for the rasterizer:
helpers
typedef float* point_t;
typedef float* triangle_t;
typedef struct edge {
point_t tail, tip;
float step_x, step_y;
int is_top_left;
} edge_t;
/* ... */
/* tail is the begining of the edge (i.e a), tip is the end of the edge (b) and c is variable */
static float init_edge(edge_t* edge, point_t tail, point_t tip, point_t origin) {
edge->tail = tail;
edge->tip = tip;
edge->is_top_left = is_top_left(tail, tip);
float A = tip[1] - tail[1];
float B = tail[0] - tip[0];
float C = tip[0] * tail[1] - tail[0] * tip[1];
/* step sizes */
edge->step_x = A;
edge->step_y = B;
/* edge function output at origin */
return A * origin[0] + B * origin[1] + C;
}
static float det(point_t a, point_t b, point_t c) {
return (b[0] - a[0]) * (a[1] - c[1]) - (a[1] - b[1]) * (c[0] - a[0]);
}
draw_triangle
void draw_triangle(sr_pipeline_t* pipeline, triangle_t triangle) {
/* orient triangle ccw */
point_t v0 = (point_t)malloc(sizeof(float) * pipeline->num_attr);
point_t v1 = (point_t)malloc(sizeof(float) * pipeline->num_attr);
point_t v2 = (point_t)malloc(sizeof(float) * pipeline->num_attr);
memcpy(v0, triangle, sizeof(float) * pipeline->num_attr);
memcpy(v1, triangle + pipeline->num_attr, sizeof(float) * pipeline->num_attr);
memcpy(v2, triangle + (2 * pipeline->num_attr), sizeof(float) * pipeline->num_attr);
orient_ccw(&v0, &v1, &v2);
/* find bounding box */
float min_x = /* ... */;
float min_y = /* ... */;
float max_x = /* ... */;
float max_y = /* ... */;
/* store current point */
point_t p = (point_t)calloc(pipeline->num_attr, sizeof(float));
p[0] = min_x;
p[1] = min_y;
/* grab edge information */
edge_t e01, e12, e20;
float w0 = init_edge(&e12, v1, v2, p);
float w1 = init_edge(&e20, v2, v0, p);
float w2 = init_edge(&e01, v0, v1, p);
/* rasterize */
for (p[1] = min_y; p[1] <= max_y; p[1]++) {
for (p[0] = min_x; p[0] <= max_x; p[0]++) {
/* determinant calculated at every step (I suspect these are correct) */
float s0 = det(v1, v2, p);
float s1 = det(v2, v0, p);
float s2 = det(v0, v1, p);
if ( (s0 >= 0) && (s1 >= 0) && (s2 >= 0) ) {
draw_point(pipeline, p);
}
w0 += e12.step_x;
w1 += e20.step_x;
w2 += e01.step_x;
}
w0 += e12.step_y;
w1 += e20.step_y;
w2 += e01.step_y;
}
free(v0);
free(v1);
free(v2);
free(p);
}
Code and functions that I have omitted I have verified work correctly.
To reiterate, my question is why are the values w0, w1, w2 not the same as s0, s1, s2 as they should be?
Any help is appreciated, thank you!

Answer 1

Rather than multiply every y value in the formula by -1, replace them with (top - y).