Add checkers pattern

orx-fx/src/main/kotlin/patterns/Checkers.kt

@@ -0,0 +1,25 @@
+package org.openrndr.extra.fx.patterns
+
+import org.openrndr.color.ColorRGBa
+import org.openrndr.draw.Filter
+import org.openrndr.draw.Shader
+import org.openrndr.extra.fx.filterFragmentCode
+import org.openrndr.extra.parameters.Description
+import org.openrndr.extra.parameters.DoubleParameter
+
+@Description("Checkers pattern")
+class Checkers : Filter(Shader.createFromCode(Filter.filterVertexCode, filterFragmentCode("patterns/checkers.frag"))) {
+    var background: ColorRGBa by parameters
+    var foreground: ColorRGBa by parameters
+    @DoubleParameter("size", 0.0, 1.0)
+    var size: Double by parameters
+    @DoubleParameter("opacity", 0.0, 1.0)
+    var opacity: Double by parameters
+
+    init {
+        size = 1.0/64.0
+        opacity = 1.0
+        foreground = ColorRGBa.WHITE.shade(0.9)
+        background = ColorRGBa.WHITE.shade(0.8)
+    }
+}

orx-fx/src/main/resources/org/openrndr/extra/fx/gl3/patterns/checkers.frag

@@ -0,0 +1,36 @@
+#version 330
+
+in vec2 v_texCoord0;
+uniform vec4 foreground;
+uniform vec4 background;
+uniform vec2 targetSize;
+uniform float size;
+uniform float opacity;
+out vec4 o_color;
+void main() {
+    float r = targetSize.x/targetSize.y;
+    vec2 uv = v_texCoord0-vec2(0.5);
+    uv.x *= r;
+
+    vec2 cell = (uv / size);
+    ivec2 cellIndex = ivec2(floor(cell));
+    vec2 cellUV = cell - cellIndex;
+
+    int c = (cellIndex.x + cellIndex.y) % 2;
+    vec2 w = fwidth(cell);
+
+    vec4 ca;
+    vec4 cb;
+    if (c == 0) {
+        ca = background;
+        cb = foreground;
+    } else {
+        ca = foreground;
+        cb = background;
+    }
+    float s = w.x;
+    float fx = smoothstep(s, 0.0, cellUV.x) + smoothstep(1.0-s, 1.0, cellUV.x);
+    float fy = smoothstep(s, 0.0, cellUV.y) + smoothstep(1.0-s, 1.0, cellUV.y);
+
+    o_color = mix(ca, cb, min(0.5, fx*0.5+ fy*0.5)) * opacity;
+}