Additional resources from the Raylib repo.

Examples/resources/shaders/glsl120/cubemap.fs

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+#version 120
+
+// Input vertex attributes (from vertex shader)
+varying vec3 fragPosition;
+
+// Input uniform values
+uniform sampler2D equirectangularMap;
+
+vec2 SampleSphericalMap(vec3 v)
+{
+    vec2 uv = vec2(atan(v.z, v.x), asin(v.y));
+    uv *= vec2(0.1591, 0.3183);
+    uv += 0.5;
+    return uv;
+}
+
+void main()
+{
+    // Normalize local position
+    vec2 uv = SampleSphericalMap(normalize(fragPosition));
+
+    // Fetch color from texture map
+    vec3 color = texture2D(equirectangularMap, uv).rgb;
+
+    // Calculate final fragment color
+    gl_FragColor = vec4(color, 1.0);
+}