WebGL / Introduced the WebGLRenderTarget class

This utility class simplifies rendering to a texture & reading the
results of the render.

It also allows clearing its content before a new render.

src/ol/webgl/RenderTarget.js

@@ -0,0 +1,124 @@
+/**
+ * A wrapper class to simplify rendering to a texture instead of the final canvas
+ * @module ol/webgl/RenderTarget
+ */
+import {equals} from '../array.js';
+
+
+/**
+ * @classdesc
+ * This class is a wrapper around the association of both a `WebGLTexture` and a `WebGLFramebuffer` instances,
+ * simplifying initialization and binding for rendering.
+ * @api
+ */
+class WebGLRenderTarget {
+
+  /**
+   * @param {import("./Helper.js").default} helper WebGL helper; mandatory.
+   * @param {Array<number>} [opt_size] Expected size of the render target texture; note: this can be changed later on.
+   */
+  constructor(helper, opt_size) {
+    /**
+     * @private
+     * @type {import("./Helper.js").default}
+     */
+    this.helper_ = helper;
+    const gl = helper.getGL();
+
+    /**
+     * @private
+     * @type {WebGLTexture}
+     */
+    this.texture_ = gl.createTexture();
+
+    /**
+     * @private
+     * @type {WebGLFramebuffer}
+     */
+    this.framebuffer_ = gl.createFramebuffer();
+
+    /**
+     * @type {Array<number>}
+     * @private
+     */
+    this.size_ = opt_size || [1, 1];
+
+    /**
+     * @type {Uint8Array}
+     * @private
+     */
+    this.data_ = new Uint8Array(1);
+
+    this.updateSize_();
+  }
+
+  /**
+   * Changes the size of the render target texture. Note: will do nothing if the size
+   * is already the same.
+   * @param {Array<number>} size Expected size of the render target texture
+   * @api
+   */
+  setSize(size) {
+    if (equals(size, this.size_)) {
+      return;
+    }
+    this.size_[0] = size[0];
+    this.size_[1] = size[1];
+    this.updateSize_();
+  }
+
+  /**
+   * Returns the size of the render target texture
+   * @return {Array<number>} Size of the render target texture
+   * @api
+   */
+  getSize() {
+    return this.size_;
+  }
+
+  /**
+   * Returns the content of the render target texture as raw data (series of r,g,b,a values)
+   * @return {Uint8Array} Integer array of color values
+   * @api
+   */
+  read() {
+    const size = this.size_;
+    const gl = this.helper_.getGL();
+
+    gl.bindFramebuffer(gl.FRAMEBUFFER, this.framebuffer_);
+    gl.readPixels(0, 0, size[0], size[1], gl.RGBA, gl.UNSIGNED_BYTE, this.data_);
+    return this.data_;
+  }
+
+  /**
+   * @return {WebGLTexture} Texture to render to
+   */
+  getTexture() {
+    return this.texture_;
+  }
+
+  /**
+   * @return {WebGLFramebuffer} Frame buffer of the render target
+   */
+  getFramebuffer() {
+    return this.framebuffer_;
+  }
+
+  /**
+   * @private
+   */
+  updateSize_() {
+    const size = this.size_;
+    const gl = this.helper_.getGL();
+
+    this.texture_ = this.helper_.createTexture(size, null, this.texture_);
+
+    gl.bindFramebuffer(gl.FRAMEBUFFER, this.framebuffer_);
+    gl.viewport(0, 0, size[0], size[1]);
+    gl.framebufferTexture2D(gl.FRAMEBUFFER, gl.COLOR_ATTACHMENT0, gl.TEXTURE_2D, this.texture_, 0);
+
+    this.data_ = new Uint8Array(size[0] * size[1] * 4);
+  }
+}
+
+export default WebGLRenderTarget;