Additional resources from the Raylib repo.

Examples/resources/shaders/glsl100/cel.fs

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+#version 100
+
+precision mediump float;
+
+varying vec3 fragPosition;
+varying vec2 fragTexCoord;
+varying vec4 fragColor;
+varying vec3 fragNormal;
+
+uniform sampler2D texture0;
+uniform vec4 colDiffuse;
+uniform vec3 viewPos;
+uniform float numBands;
+
+struct Light {
+    int enabled;
+    int type;
+    vec3 position;
+    vec3 target;
+    vec4 color;
+};
+uniform Light lights[4];
+
+void main()
+{
+    vec4 texColor = texture2D(texture0, fragTexCoord);
+    vec3 baseColor = texColor.rgb * fragColor.rgb * colDiffuse.rgb;
+    vec3 norm = normalize(fragNormal);
+
+    float lightAccum = 0.08; // ambient floor
+
+    for (int i = 0; i < 4; i++)
+    {
+        if (lights[i].enabled == 1)  // no continue in GLSL ES 1.0
+        {
+            vec3 lightDir;
+            if (lights[i].type == 0)
+            {
+                // Directional: direction is from position toward target.
+                lightDir = normalize(lights[i].position - lights[i].target);
+            }
+            else
+            {
+                // Point: direction from surface to light.
+                lightDir = normalize(lights[i].position - fragPosition);
+            }
+
+            float NdotL = max(dot(norm, lightDir), 0.0);
+
+            // Quantize NdotL into numBands discrete steps.
+            float quantized = min(floor(NdotL * numBands), numBands - 1.0) / (numBands - 1.0);
+            lightAccum += quantized * lights[i].color.r;
+        }
+    }
+
+    lightAccum = clamp(lightAccum, 0.0, 1.0);
+    gl_FragColor = vec4(baseColor * lightAccum, texColor.a * colDiffuse.a);
+}