tweak renderer API

resources/shaders/source/msdf.frag.glsl

@@ -0,0 +1,35 @@
+#version 450
+
+layout(set = 3, binding = 0) uniform Viewport {
+    ivec2 view;
+};
+layout(set = 2, binding = 0) uniform sampler2D tx;
+
+const float PX_RANGE = 4.0f;
+
+layout(location = 0) in vec2 uv;
+layout(location = 1) in vec4 color;
+
+layout(location = 0) out vec4 fragColor;
+
+float screen_px_range(vec2 uv) {
+    vec2 unit_range = vec2(PX_RANGE)/vec2(textureSize(tx, 0));
+    vec2 texel_size = vec2(1.0)/fwidth(uv);
+    return max(0.5*dot(unit_range, texel_size), 1.0);
+}
+
+float median(float r, float g, float b) {
+	return max(min(r, g), min(max(r, g), b));
+}
+
+void main() {
+    ivec2 ts = textureSize(tx, 0);
+    vec2 fts = vec2(float(ts.x), float(ts.y));
+    vec2 real_uv = uv / fts;
+
+	vec3 msd = texture(tx, real_uv).rgb;
+	float sd = median(msd.r, msd.g, msd.b);
+	float distance = screen_px_range(real_uv)*(sd - 0.5);
+	float opacity = clamp(distance + 0.5, 0.0, 1.0);
+	fragColor = color * opacity;
+}