Add rainbow to the fragment shader

shaders/font.frag

@@ -14,6 +14,7 @@
 
 uniform sampler2D font;
 uniform float time;
+uniform vec2 resolution;
 
 in vec2 uv;
 flat in int glyph_ch;
@@ -25,6 +26,11 @@ float map01(float x)
     return (x + 1) / 2.0;
 }
 
+vec3 hsl2rgb(vec3 c) {
+    vec3 rgb = clamp(abs(mod(c.x*6.0+vec3(0.0,4.0,2.0),6.0)-3.0)-1.0, 0.0, 1.0);
+    return c.z + c.y * (rgb-0.5)*(1.0-abs(2.0*c.z-1.0));
+}
+
 void main() {
     int ch = glyph_ch;
     if (!(ASCII_DISPLAY_LOW <= ch && ch <= ASCII_DISPLAY_HIGH)) {
@@ -39,10 +45,7 @@ void main() {
     vec2 t = pos + size * uv;
 
     vec4 tc = texture(font, t);
-    vec4 rainbow = vec4(
-        map01(sin(time + uv.x)),
-        map01(cos(time + uv.y)),
-        map01(sin(time + uv.x + uv.y)),
-        1.0);
+    vec2 frag_uv = gl_FragCoord.xy / resolution;
+    vec4 rainbow = vec4(hsl2rgb(vec3((time + frag_uv.x + frag_uv.y), 0.5, 0.5)), 1.0);
     gl_FragColor = glyph_bg_color * (1.0 - tc.x) + tc.x * glyph_fg_color * rainbow;
 }