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Retain subdivision results

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Don't recompute the parameters from quadratic subdivision, but rather
retain them across the two phases (summing the subdivision estimate, and
generating the subdivisions). The motivation for this is that the values
were subtly different (differing by 1 or 2 least signficant bits) across
the two phases. It *might* also be faster depending on ALU/memory
relative performance.

Fixes #107
This commit is contained in:
Raph Levien 2021-07-15 11:18:48 -07:00
parent a542833646
commit 29a8975a9a
2 changed files with 9 additions and 1 deletions
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10
piet-gpu/shader/path_coarse.comp
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@ -30,6 +30,7 @@ layout(set = 0, binding = 1) readonly buffer ConfigBuf {
#define Q_ACCURACY (ACCURACY * 0.1) #define Q_ACCURACY (ACCURACY * 0.1)
#define REM_ACCURACY (ACCURACY - Q_ACCURACY) #define REM_ACCURACY (ACCURACY - Q_ACCURACY)
#define MAX_HYPOT2 (432.0 * Q_ACCURACY * Q_ACCURACY) #define MAX_HYPOT2 (432.0 * Q_ACCURACY * Q_ACCURACY)
#define MAX_QUADS 16
vec2 eval_quad(vec2 p0, vec2 p1, vec2 p2, float t) { vec2 eval_quad(vec2 p0, vec2 p1, vec2 p2, float t) {
float mt = 1.0 - t; float mt = 1.0 - t;
@ -69,6 +70,9 @@ SubdivResult estimate_subdiv(vec2 p0, vec2 p1, vec2 p2, float sqrt_tol) {
float x0 = (d01.x * dd.x + d01.y * dd.y) / cross; float x0 = (d01.x * dd.x + d01.y * dd.y) / cross;
float x2 = (d12.x * dd.x + d12.y * dd.y) / cross; float x2 = (d12.x * dd.x + d12.y * dd.y) / cross;
float scale = abs(cross / (length(dd) * (x2 - x0))); float scale = abs(cross / (length(dd) * (x2 - x0)));
if (abs(cross) < 1e-9) {
//return SubdivResult(0.0, 0.0, 0.0);
}
float a0 = approx_parabola_integral(x0); float a0 = approx_parabola_integral(x0);
float a2 = approx_parabola_integral(x2); float a2 = approx_parabola_integral(x2);
@ -113,6 +117,8 @@ void main() {
float err = err_v.x * err_v.x + err_v.y * err_v.y; float err = err_v.x * err_v.x + err_v.y * err_v.y;
// The number of quadratics. // The number of quadratics.
uint n_quads = max(uint(ceil(pow(err * (1.0 / MAX_HYPOT2), 1.0 / 6.0))), 1); uint n_quads = max(uint(ceil(pow(err * (1.0 / MAX_HYPOT2), 1.0 / 6.0))), 1);
n_quads = min(n_quads, MAX_QUADS);
SubdivResult keep_params[MAX_QUADS];
// Iterate over quadratics and tote up the estimated number of segments. // Iterate over quadratics and tote up the estimated number of segments.
float val = 0.0; float val = 0.0;
vec2 qp0 = cubic.p0; vec2 qp0 = cubic.p0;
@ -123,6 +129,7 @@ void main() {
vec2 qp1 = eval_cubic(cubic.p0, cubic.p1, cubic.p2, cubic.p3, t - 0.5 * step); vec2 qp1 = eval_cubic(cubic.p0, cubic.p1, cubic.p2, cubic.p3, t - 0.5 * step);
qp1 = 2.0 * qp1 - 0.5 * (qp0 + qp2); qp1 = 2.0 * qp1 - 0.5 * (qp0 + qp2);
SubdivResult params = estimate_subdiv(qp0, qp1, qp2, sqrt(REM_ACCURACY)); SubdivResult params = estimate_subdiv(qp0, qp1, qp2, sqrt(REM_ACCURACY));
keep_params[i] = params;
val += params.val; val += params.val;
qp0 = qp2; qp0 = qp2;
@ -144,12 +151,13 @@ void main() {
vec2 qp2 = eval_cubic(cubic.p0, cubic.p1, cubic.p2, cubic.p3, t); vec2 qp2 = eval_cubic(cubic.p0, cubic.p1, cubic.p2, cubic.p3, t);
vec2 qp1 = eval_cubic(cubic.p0, cubic.p1, cubic.p2, cubic.p3, t - 0.5 * step); vec2 qp1 = eval_cubic(cubic.p0, cubic.p1, cubic.p2, cubic.p3, t - 0.5 * step);
qp1 = 2.0 * qp1 - 0.5 * (qp0 + qp2); qp1 = 2.0 * qp1 - 0.5 * (qp0 + qp2);
SubdivResult params = estimate_subdiv(qp0, qp1, qp2, sqrt(REM_ACCURACY)); SubdivResult params = keep_params[i];
float u0 = approx_parabola_inv_integral(params.a0); float u0 = approx_parabola_inv_integral(params.a0);
float u2 = approx_parabola_inv_integral(params.a2); float u2 = approx_parabola_inv_integral(params.a2);
float uscale = 1.0 / (u2 - u0); float uscale = 1.0 / (u2 - u0);
float target = float(n_out) * v_step; float target = float(n_out) * v_step;
while (n_out == n || target < val_sum + params.val) { while (n_out == n || target < val_sum + params.val) {
if (n_out == 1000) break;
vec2 p1; vec2 p1;
if (n_out == n) { if (n_out == n) {
p1 = cubic.p3; p1 = cubic.p3;

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piet-gpu/shader/path_coarse.spv
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