Additional resources from the Raylib repo.

Examples/resources/shaders/glsl120/skybox.fs

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+#version 120
+
+// Input vertex attributes (from vertex shader)
+varying vec3 fragPosition;
+
+// Input uniform values
+uniform samplerCube environmentMap;
+uniform bool vflipped;
+uniform bool doGamma;
+
+void main()
+{
+    // Fetch color from texture map
+    vec4 texelColor = vec4(0.0);
+
+    if (vflipped) texelColor = textureCube(environmentMap, vec3(fragPosition.x, -fragPosition.y, fragPosition.z));
+    else texelColor = textureCube(environmentMap, fragPosition);
+
+    vec3 color = vec3(texelColor.x, texelColor.y, texelColor.z);
+
+    if (doGamma) // Apply gamma correction
+    {
+        color = color/(color + vec3(1.0));
+        color = pow(color, vec3(1.0/2.2));
+    }
+
+    // Calculate final fragment color
+    gl_FragColor = vec4(color, 1.0);
+}