Add FastIntegralImage documentation

orx-integral-image/README.md

@@ -10,3 +10,52 @@ val integralImage = IntegralImage.fromColorBufferShadow(image.shadow)
 // -- the sum for a given area can be queried using
 val sum = integralImage.sum(IntRectangle(20, 20, 100, 100))
 ```
+
+## Fast Integral Image
+
+Since v0.0.20 orx-integral-image comes with `FastIntegralImage` which calculates integral images fully on the GPU.
+
+```kotlin
+import org.openrndr.application
+import org.openrndr.color.ColorRGBa
+import org.openrndr.draw.*
+import org.openrndr.extra.integralimage.*
+
+fun main(args: Array<String>) = application {
+    configure {
+        width = 1024
+        height = 1024
+    }
+    program {
+        val fii = FastIntegralImage()
+        val target = colorBuffer(1024, 512, 1.0, ColorFormat.RGBa, ColorType.FLOAT32)
+        val rt = renderTarget(1024, 512) {
+            colorBuffer()
+        }
+        extend {
+            drawer.background(ColorRGBa.PINK)
+            drawer.isolatedWithTarget(rt) {
+                drawer.ortho(rt)
+                drawer.background(ColorRGBa.BLACK)
+                drawer.fill = ColorRGBa.PINK.shade(1.0)
+                drawer.circle(mouse.position, 128.0)
+            }
+            fii.apply(rt.colorBuffer(0),target)
+
+            // -- here we sample from the integral image
+            drawer.shadeStyle = shadeStyle {
+                fragmentTransform = """
+                    float w = 128.0;
+                    vec2 step = 1.0 / textureSize(image, 0);
+                    vec4 t11 = texture(image, va_texCoord0 + step * vec2(w,w));
+                    vec4 t01 = texture(image, va_texCoord0 + step * vec2(-w,w));
+                    vec4 t00 = texture(image, va_texCoord0 + step * vec2(-w,-w));
+                    vec4 t10 = texture(image, va_texCoord0 + step * vec2(w,-w));
+                    x_fill = (t11 - t01 - t10 + t00) / (w*w);
+                """.trimIndent()
+            }
+            drawer.image(target)
+        }
+    }
+}
+```