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// VERTEX_SHADER
uniform vec4 u_points;
_NOPERSPECTIVE_ _OUT_ vec4 info;
void main() {
gl_Position = u_projection * (u_modelview * vec4(aPosition, 0.0, 1.0));
vec2 mv0 = u_modelview[0].xy;
vec2 mv1 = u_modelview[1].xy;
vec2 offset = aPosition - u_points.xy;
vec2 coord = vec2(dot(mv0, offset),
dot(mv1, offset));
// Original equation:
// VS | maxDist = length(end - start);
// VS | gradient = end - start;
// VS | gradientLength = length(gradient);
// FS | pos = frag_coord - start
// FS | proj = (dot(gradient, pos) / (gradientLength * gradientLength)) * gradient
// FS | offset = length(proj) / maxDist
// Simplified formula derivation:
// 1. Notice that maxDist = gradientLength:
// offset = length(proj) / gradientLength
// 2. Let gnorm = gradient / gradientLength, then:
// proj = (dot(gnorm * gradientLength, pos) / (gradientLength * gradientLength)) * (gnorm * gradientLength) =
// = dot(gnorm, pos) * gnorm
// 3. Since gnorm is unit length then:
// length(proj) = length(dot(gnorm, pos) * gnorm) = dot(gnorm, pos)
// 4. We can avoid the FS division by passing a scaled pos from the VS:
// offset = dot(gnorm, pos) / gradientLength = dot(gnorm, pos / gradientLength)
// 5. 1.0 / length(gradient) is inversesqrt(dot(gradient, gradient)) in GLSL
vec2 gradient = vec2(dot(mv0, u_points.zw),
dot(mv1, u_points.zw));
float rcp_gradient_length = inversesqrt(dot(gradient, gradient));
info = rcp_gradient_length * vec4(coord, gradient);
}
// FRAGMENT_SHADER:
#ifdef GSK_LEGACY
uniform int u_num_color_stops;
#else
uniform highp int u_num_color_stops; // Why? Because it works like this.
#endif
uniform float u_color_stops[6 * 5];
uniform bool u_repeat;
_NOPERSPECTIVE_ _IN_ vec4 info;
float get_offset(int index) {
return u_color_stops[5 * index];
}
vec4 get_color(int index) {
int base = 5 * index + 1;
return vec4(u_color_stops[base],
u_color_stops[base + 1],
u_color_stops[base + 2],
u_color_stops[base + 3]);
}
void main() {
float offset = dot(info.xy, info.zw);
if (u_repeat) {
offset = fract(offset);
}
if (offset < get_offset(0)) {
gskSetOutputColor(gsk_scaled_premultiply(get_color(0), u_alpha));
return;
}
int n = u_num_color_stops - 1;
for (int i = 0; i < n; i++) {
float curr_offset = get_offset(i);
float next_offset = get_offset(i + 1);
if (offset >= curr_offset && offset < next_offset) {
float f = (offset - curr_offset) / (next_offset - curr_offset);
vec4 curr_color = gsk_premultiply(get_color(i));
vec4 next_color = gsk_premultiply(get_color(i + 1));
vec4 color = mix(curr_color, next_color, f);
gskSetOutputColor(color * u_alpha);
return;
}
}
gskSetOutputColor(gsk_scaled_premultiply(get_color(n), u_alpha));
}
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