+ int num_patches = textureSize(flow_tex, 0).x * textureSize(flow_tex, 0).y;
+ int patch_layer = gl_InstanceID / num_patches;
+ int patch_x = gl_InstanceID % textureSize(flow_tex, 0).x;
+ int patch_y = (gl_InstanceID % num_patches) / textureSize(flow_tex, 0).x;