Additional resources from the Raylib repo.

Examples/resources/shaders/glsl330/cel.fs

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+#version 330
+
+in vec3 fragPosition;
+in vec2 fragTexCoord;
+in vec4 fragColor;
+in vec3 fragNormal;
+
+// Raylib standard uniforms
+uniform sampler2D texture0;
+uniform vec4 colDiffuse;
+
+// View position for future specular / fresnel use.
+uniform vec3 viewPos;
+
+// Number of discrete toon bands (2 = hard binary, 10 = default, 20 = near-smooth).
+uniform float numBands;
+
+// rlights.h compatible light block.
+struct Light {
+    int  enabled;
+    int  type; // 0 = directional, 1 = point
+    vec3 position;
+    vec3 target;
+    vec4 color;
+    float attenuation;
+};
+uniform Light lights[4];
+
+out vec4 finalColor;
+
+void main() {
+    vec4 texColor = texture(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 == 0) continue;
+
+        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.
+        // min() guards against NdotL == 1.0 producing an out-of-range index.
+        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);
+    finalColor = vec4(baseColor * lightAccum, texColor.a * colDiffuse.a);
+}