Additional resources from the Raylib repo.

Examples/resources/shaders/glsl120/sdf.fs

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+#version 120
+
+// Input vertex attributes (from vertex shader)
+varying vec2 fragTexCoord;
+varying vec4 fragColor;
+
+// Input uniform values
+uniform sampler2D texture0;
+uniform vec4 colDiffuse;
+
+// NOTE: Add your custom variables here
+const float smoothing = 1.0/16.0;
+
+void main()
+{
+    // Texel color fetching from texture sampler
+    // NOTE: Calculate alpha using signed distance field (SDF)
+    float distance = texture2D(texture0, fragTexCoord).a;
+    float alpha = smoothstep(0.5 - smoothing, 0.5 + smoothing, distance);
+
+    // Calculate final fragment color
+    gl_FragColor = vec4(fragColor.rgb, fragColor.a*alpha);
+}