feat: Add rainbow color effect to fragment shader and update vertex shader to pass world position

shaders/cube.frag

@@ -1,8 +1,48 @@
 #version 450
 
 layout(location = 0) in vec3 fragColor;
+layout(location = 1) in vec3 fragWorldPos;
 layout(location = 0) out vec4 outColor;
 
-void main() {
-    outColor = vec4(fragColor, 1.0);
+vec3 RainbowBand(float t) {
+    t = clamp(t, 0.0, 1.0);
+    float scaled = t * 5.0;
+    int index = int(floor(scaled));
+    float blend = fract(scaled);
+
+    vec3 red = vec3(0.91, 0.19, 0.21);
+    vec3 orange = vec3(0.99, 0.49, 0.09);
+    vec3 yellow = vec3(0.99, 0.86, 0.22);
+    vec3 green = vec3(0.16, 0.74, 0.39);
+    vec3 blue = vec3(0.24, 0.48, 0.88);
+    vec3 purple = vec3(0.56, 0.25, 0.75);
+
+    vec3 a = red;
+    vec3 b = orange;
+    if (index == 1) {
+        a = orange;
+        b = yellow;
+    } else if (index == 2) {
+        a = yellow;
+        b = green;
+    } else if (index == 3) {
+        a = green;
+        b = blue;
+    } else if (index == 4) {
+        a = blue;
+        b = purple;
+    } else if (index >= 5) {
+        a = purple;
+        b = purple;
+    }
+
+    return mix(a, b, blend);
+}
+
+void main() {
+    float bandPos = (fragWorldPos.y + 1.0) * 0.5;
+    float diagonal = (fragWorldPos.x + fragWorldPos.z) * 0.15;
+    vec3 rainbow = RainbowBand(bandPos + diagonal);
+    vec3 shaded = mix(rainbow, fragColor, 0.08);
+    outColor = vec4(shaded, 1.0);
 }

shaders/cube.vert

@@ -4,6 +4,7 @@ layout(location = 0) in vec3 inPos;
 layout(location = 1) in vec3 inColor;
 
 layout(location = 0) out vec3 fragColor;
+layout(location = 1) out vec3 fragWorldPos;
 
 layout(push_constant) uniform PushConstants {
     mat4 model;
@@ -12,5 +13,7 @@ layout(push_constant) uniform PushConstants {
 
 void main() {
     fragColor = inColor;
-    gl_Position = pushConstants.viewProj * pushConstants.model * vec4(inPos, 1.0);
+    vec4 worldPos = pushConstants.model * vec4(inPos, 1.0);
+    fragWorldPos = worldPos.xyz;
+    gl_Position = pushConstants.viewProj * worldPos;
 }