Combine render passes where feasible

src/platform/graphics/render-pass.js

@@ -126,6 +126,24 @@ class RenderPass {
      */
     _enabled = true;
 
+    /**
+     * True if the render pass start is skipped. This means the render pass is merged into the
+     * previous one.
+     *
+     * @type {boolean}
+     * @private
+     */
+    _skipStart = false;
+
+    /**
+     * True if the render pass end is skipped. This means the following render pass is merged into
+     * this one.
+     *
+     * @type {boolean}
+     * @private
+     */
+    _skipEnd = false;
+
     /**
      * True if the render pass is enabled and execute function will be called. Note that before and
      * after functions are called regardless of this flag.
@@ -414,13 +432,13 @@ class RenderPass {
 
             if (this.executeEnabled) {
 
-                if (realPass) {
+                if (realPass && !this._skipStart) {
                     device.startRenderPass(this);
                 }
 
                 this.execute();
 
-                if (realPass) {
+                if (realPass && !this._skipEnd) {
                     device.endRenderPass(this);
                 }
             }
@@ -447,8 +465,9 @@ class RenderPass {
                 ` ${rt.width} x ${rt.height}` +
                 `${(this.samples > 0 ? ` samples: ${this.samples}` : '')}`;
 
+            const indexString = this._skipStart ? '++' : index.toString().padEnd(2, ' ');
             Debug.trace(TRACEID_RENDER_PASS,
-                `${index.toString().padEnd(2, ' ')}: ${this.name.padEnd(20, ' ')}` +
+                `${indexString}: ${this.name.padEnd(20, ' ')}` +
                         `${this.executeEnabled ? '' : ' DISABLED '}${
                             rtInfo.padEnd(30)}`);
 

src/scene/frame-graph.js

@@ -90,6 +90,41 @@ class FrameGraph {
             }
         }
 
+        // merge passes if possible
+        for (let i = 0; i < renderPasses.length - 1; i++) {
+            const firstPass = renderPasses[i];
+            const firstRT = firstPass.renderTarget;
+            const secondPass = renderPasses[i + 1];
+            const secondRT = secondPass.renderTarget;
+
+            // if the render targets are different, we can't merge the passes
+            // also only merge passes that have a render target
+            if (firstRT !== secondRT || firstRT === undefined) {
+                continue;
+            }
+
+            // do not merge if the second pass clears any of the attachments
+            if (secondPass.depthStencilOps.clearDepth ||
+                secondPass.depthStencilOps.clearStencil ||
+                secondPass.colorArrayOps.some(colorOps => colorOps.clear)) {
+                continue;
+            }
+
+            // first pass cannot contain after passes
+            if (firstPass.afterPasses.length > 0) {
+                continue;
+            }
+
+            // second pass cannot contain before passes
+            if (secondPass.beforePasses.length > 0) {
+                continue;
+            }
+
+            // merge the passes
+            firstPass._skipEnd = true;
+            secondPass._skipStart = true;
+        }
+
         // Walk over render passes to find passes rendering to the same cubemap texture.
         // If those passes are separated only by passes not requiring cubemap (shadows ..),
         // we skip the mipmap generation till the last rendering to the cubemap, to avoid