[openrndr-demos] Add demo for circle billboards

openrndr-demos/src/demo/kotlin/DemoBillboardCircles01.kt

@@ -0,0 +1,61 @@
+// Demonstration of circles that always face the camera
+
+import org.openrndr.WindowMultisample
+import org.openrndr.application
+import org.openrndr.color.ColorRGBa
+import org.openrndr.draw.*
+import org.openrndr.extra.noise.uniformRing
+import org.openrndr.extras.camera.Orbital
+import org.openrndr.math.Vector2
+import org.openrndr.math.Vector3
+
+
+fun main() = application {
+
+    configure {
+        multisample = WindowMultisample.SampleCount(8)
+    }
+    program {
+        extend(Orbital())
+
+
+        val circlePositions = vertexBuffer(vertexFormat {
+            attribute("position", VertexElementType.VECTOR3_FLOAT32)
+            attribute("scale", VertexElementType.FLOAT32)
+        }, 1000)
+
+        circlePositions.put {
+            for (i in 0 until circlePositions.vertexCount) {
+                write(Vector3.uniformRing(0.0, 3.0))
+                write(Math.random().toFloat()*0.1f)
+            }
+        }
+
+
+        extend {
+            drawer.perspective(90.0, width*1.0/height*1.0, 0.1, 100.0)
+
+            drawer.fill = ColorRGBa.PINK
+            drawer.stroke = ColorRGBa.GREEN
+            drawer.strokeWeight = 0.05
+            drawer.drawStyle.alphaToCoverage = true
+
+            drawer.depthWrite = true
+            drawer.depthTestPass = DepthTestPass.LESS_OR_EQUAL
+
+            drawer.shadeStyle = shadeStyle {
+                vertexTransform = """
+                    vec3 viewOffset = (x_viewMatrix * x_modelMatrix * vec4(i_position, 1.0)).xyz;
+                    vec2 i = vec2(0.0, 1.0);
+                    x_viewMatrix = mat4(i.yxxx, i.xyxx, i.xxyx, i.xxxy);
+                    x_modelMatrix = mat4(i.yxxx, i.xyxx, i.xxyx, i.xxxy);
+                    x_position = viewOffset + vec3(a_position.xy * i_scale, 0.0);
+                    vi_radius = vec2(i_scale);
+                """.trimIndent()
+                attributes(circlePositions)
+            }
+
+            drawer.circles((0 until circlePositions.vertexCount).map {  Vector2.ZERO }, 0.0)
+        }
+    }
+}