Added "clearEachPass" option to SceneJS.Scene

src/core/engine.js

@@ -18,11 +18,18 @@ var SceneJS_Engine = function (json, options) {
 
     /**
      * Number of times the scene is drawn each time it's rendered.
-     * <p>This is useful for when we need to do things like render for left and right eyes.
+     * <p>This is useful for when we need to do things like render for left and right eyes.</p>
      * @type {*|number}
      */
     this._numPasses = json.numPasses || 1;
 
+    /**
+     * When doing multiple passes per frame, specifies whether to clear the
+     * canvas before each pass (true) or just before the first pass (false, default).
+     * @type Boolean
+     */
+    this._clearEachPass = !!json.clearEachPass;
+
     /**
      * Canvas and GL context for this engine
      */
@@ -193,7 +200,7 @@ var SceneJS_Engine = function (json, options) {
 
 /**
  * Sets the number of times the scene is drawn on each render.
- * <p>This is useful for when we need to do things like render for left and right eyes.
+ * <p>This is useful for when we need to do things like render for left and right eyes.</p>
  * @param {Number} numPasses The number of times the scene is drawn on each frame.
  * @see #getTagMask
  * @see SceneJS.Tag
@@ -202,6 +209,20 @@ SceneJS_Engine.prototype.setNumPasses = function (numPasses) {
     this._numPasses = numPasses;
 };
 
+/**
+ *  When doing multiple passes per frame, specifies whether to clear the
+ * canvas before each pass (true) or just before the first pass (false).
+ *
+ * <p>This is useful for when we need to do things like render a separate pass to a stereo framebuffer for left and right eyes,
+ * where we want to clear the buffer before each pass.</p>
+ *
+ * @param {Boolean} clearEachPass True to clear before each pass (default is false).
+ * @see SceneJS.Tag
+ */
+SceneJS_Engine.prototype.setClearEachPass = function (clearEachPass) {
+    this._clearEachPass = clearEachPass;
+};
+
 /**
  * Simulate a lost WebGL context.
  * Only works if the simulateWebGLContextLost was given as an option to the engine's constructor.
@@ -393,7 +414,7 @@ SceneJS_Engine.prototype.renderFrame = function (params) {
             // Render display graph
             // Clear buffers only on first frame
             this.display.render({
-                clear: i == 0,
+                clear: this._clearEachPass || i == 0,
                 force: force,
                 opaqueOnly: params && params.opaqueOnly
             });

src/core/scene/scene.js

@@ -98,15 +98,31 @@ SceneJS.Scene.prototype.getTagMask = function () {
 
 /**
  * Sets the number of times this scene is drawn on each render.
- * <p>This is useful for when we need to do things like render for left and right eyes.
+ *
+ * <p>This is useful for when we need to do things like render for left and right eyes.</p>
+ *
  * @param {Number} numPasses The number of times the scene is drawn on each frame.
- * @see #getTagMask
  * @see SceneJS.Tag
  */
 SceneJS.Scene.prototype.setNumPasses = function (numPasses) {
     this._engine.setNumPasses(numPasses);
 };
 
+/**
+ *  When doing multiple passes per frame, specifies whether to clear the
+ * canvas before each pass (true) or just before the first pass (false).
+ *
+ * <p>This is useful for when we need to do things like render a separate pass to a stereo framebuffer for left and right eyes,
+ * where we want to clear the buffer before each pass.</p>
+ *
+ * @param {Boolean} clearEachPass Tryu to clear before each pass (default is false).
+ * @see SceneJS.Tag
+ */
+SceneJS.Scene.prototype.setClearEachPass = function (clearEachPass) {
+    this._engine.setClearEachPass(clearEachPass);
+};
+
+
 /**
  * Render a single frame if new frame pending, or force a new frame
  * Returns true if frame rendered

src/core/scenejs.js

@@ -190,6 +190,9 @@ var SceneJS = new (function () {
      * @param {String} json JSON scene description
      * @param {*} options Optional options
      * @param {Boolean} options.simulateWebGLContextLost Set true to enable simulation of lost WebGL context (has performance impact)
+     * @param {Number} options.passes The number of times this Scene renders per frame.
+     * @param {Boolean} options.clearEachPass When doing multiple passes per frame, specifies whether to clear the
+     * canvas before each pass (true) or just before the first pass (false).
      * @returns {SceneJS.Scene} New scene
      */
     this.createScene = function (json, options) {