for (int x = y + 1; x <= R; ++x) {
vec4 sample = PREFIX(samples)[y * (R + 1) + x];
vec2 mirror_sample = vec2(sample.x, -sample.y);
vec4 local_sum = INPUT(tc - sample.xy) + INPUT(tc + sample.xy);
local_sum += INPUT(tc - mirror_sample.xy) + INPUT(tc + mirror_sample.xy);
for (int x = y + 1; x <= R; ++x) {
vec4 sample = PREFIX(samples)[y * (R + 1) + x];
vec2 mirror_sample = vec2(sample.x, -sample.y);
vec4 local_sum = INPUT(tc - sample.xy) + INPUT(tc + sample.xy);
local_sum += INPUT(tc - mirror_sample.xy) + INPUT(tc + mirror_sample.xy);