Update view's alpha and layer docs

bug:8501661

Makes the performance issues and interaction with layer type/paint
more clear.

Additionally, corrects change from 47ab7d6612e2b5b8b66fb261dafef7c91264e173
to still allow displayList alpha to override layer paint

Change-Id: Ic94d75865700820489370461cd8ac9f9077a8d90

3 files changed, 43 insertions, 11 deletions

diff --git a/libs/hwui/DisplayList.cpp b/libs/hwui/DisplayList.cpp
index d985ad0..36c95f9 100644
--- a/libs/hwui/DisplayList.cpp
+++ b/libs/hwui/DisplayList.cpp
@@ -352,7 +352,9 @@ void DisplayList::outputViewProperties(const int level) {
         }
     }
     if (mAlpha < 1) {
-        if (mCaching || !mHasOverlappingRendering) {
+        if (mCaching) {
+            ALOGD("%*sSetOverrideLayerAlpha %.2f", level * 2, "", mAlpha);
+        } else if (!mHasOverlappingRendering) {
             ALOGD("%*sScaleAlpha %.2f", level * 2, "", mAlpha);
         } else {
             int flags = SkCanvas::kHasAlphaLayer_SaveFlag;
@@ -400,7 +402,9 @@ void DisplayList::setViewProperties(OpenGLRenderer& renderer, T& handler,
         }
     }
     if (mAlpha < 1) {
-        if (mCaching || !mHasOverlappingRendering) {
+        if (mCaching) {
+            renderer.setOverrideLayerAlpha(mAlpha);
+        } else if (!mHasOverlappingRendering) {
             renderer.scaleAlpha(mAlpha);
         } else {
             // TODO: should be able to store the size of a DL at record time and not
@@ -513,6 +517,7 @@ void DisplayList::iterate(OpenGLRenderer& renderer, T& handler, const int level)
     DISPLAY_LIST_LOGD("%*sRestoreToCount %d", (level + 1) * 2, "", restoreTo);
     handler(mRestoreToCountOp->reinit(restoreTo), PROPERTY_SAVECOUNT);
     renderer.restoreToCount(restoreTo);
+    renderer.setOverrideLayerAlpha(1.0f);
 }
 
 }; // namespace uirenderer
diff --git a/libs/hwui/OpenGLRenderer.cpp b/libs/hwui/OpenGLRenderer.cpp
index 4a5785c..1138998 100644
--- a/libs/hwui/OpenGLRenderer.cpp
+++ b/libs/hwui/OpenGLRenderer.cpp
@@ -114,6 +114,7 @@ OpenGLRenderer::OpenGLRenderer():
         mCaches(Caches::getInstance()), mExtensions(Extensions::getInstance()) {
     mDrawModifiers.mShader = NULL;
     mDrawModifiers.mColorFilter = NULL;
+    mDrawModifiers.mOverrideLayerAlpha = 1.0f;
     mDrawModifiers.mHasShadow = false;
     mDrawModifiers.mHasDrawFilter = false;
 
@@ -1074,7 +1075,7 @@ void OpenGLRenderer::composeLayerRect(Layer* layer, const Rect& rect, bool swap)
             layer->setFilter(GL_LINEAR, true);
         }
 
-        float alpha = layer->getAlpha() / 255.0f * mSnapshot->alpha;
+        float alpha = getLayerAlpha(layer);
         bool blend = layer->isBlend() || alpha < 1.0f;
         drawTextureMesh(x, y, x + rect.getWidth(), y + rect.getHeight(),
                 layer->getTexture(), alpha, layer->getMode(), blend,
@@ -1112,7 +1113,7 @@ void OpenGLRenderer::composeLayerRegion(Layer* layer, const Rect& rect) {
             rects = safeRegion.getArray(&count);
         }
 
-        const float alpha = layer->getAlpha() / 255.0f * mSnapshot->alpha;
+        const float alpha = getLayerAlpha(layer);
         const float texX = 1.0f / float(layer->getWidth());
         const float texY = 1.0f / float(layer->getHeight());
         const float height = rect.getHeight();
@@ -2237,7 +2238,7 @@ status_t OpenGLRenderer::drawPatch(SkBitmap* bitmap, const int32_t* xDivs, const
         float left, float top, float right, float bottom, SkPaint* paint) {
     int alpha;
     SkXfermode::Mode mode;
-    getAlphaAndModeDirect(paint, &alpha, &mode);
+    getAlphaAndMode(paint, &alpha, &mode);
 
     return drawPatch(bitmap, xDivs, yDivs, colors, width, height, numColors,
             left, top, right, bottom, alpha, mode);
@@ -2990,7 +2991,7 @@ status_t OpenGLRenderer::drawLayer(Layer* layer, float x, float y) {
         if (layer->region.isRect()) {
             composeLayerRect(layer, layer->regionRect);
         } else if (layer->mesh) {
-            const float a = layer->getAlpha() / 255.0f * mSnapshot->alpha;
+            const float a = getLayerAlpha(layer);
             setupDraw();
             setupDrawWithTexture();
             setupDrawColor(a, a, a, a);
@@ -3446,10 +3447,24 @@ void OpenGLRenderer::resetDrawTextureTexCoords(float u1, float v1, float u2, flo
     TextureVertex::setUV(v++, u2, v2);
 }
 
-void OpenGLRenderer::getAlphaAndMode(SkPaint* paint, int* alpha, SkXfermode::Mode* mode) {
+void OpenGLRenderer::getAlphaAndMode(SkPaint* paint, int* alpha, SkXfermode::Mode* mode) const {
     getAlphaAndModeDirect(paint, alpha,  mode);
+    if (mDrawModifiers.mOverrideLayerAlpha < 1.0f) {
+        // if drawing a layer, ignore the paint's alpha
+        *alpha = mDrawModifiers.mOverrideLayerAlpha;
+    }
     *alpha *= mSnapshot->alpha;
 }
 
+float OpenGLRenderer::getLayerAlpha(Layer* layer) const {
+    float alpha;
+    if (mDrawModifiers.mOverrideLayerAlpha < 1.0f) {
+        alpha = mDrawModifiers.mOverrideLayerAlpha;
+    } else {
+        alpha = layer->getAlpha() / 255.0f;
+    }
+    return alpha * mSnapshot->alpha;
+}
+
 }; // namespace uirenderer
 }; // namespace android
diff --git a/libs/hwui/OpenGLRenderer.h b/libs/hwui/OpenGLRenderer.h
index 04a47fc..dd7a5a2 100644
--- a/libs/hwui/OpenGLRenderer.h
+++ b/libs/hwui/OpenGLRenderer.h
@@ -51,6 +51,7 @@ namespace uirenderer {
 struct DrawModifiers {
     SkiaShader* mShader;
     SkiaColorFilter* mColorFilter;
+    float mOverrideLayerAlpha;
 
     // Drop shadow
     bool mHasShadow;
@@ -275,6 +276,9 @@ public:
     virtual void resetPaintFilter();
     virtual void setupPaintFilter(int clearBits, int setBits);
 
+    // If this value is set to < 1.0, it overrides alpha set on layer (see drawBitmap, drawLayer)
+    void setOverrideLayerAlpha(float alpha) { mDrawModifiers.mOverrideLayerAlpha = alpha; }
+
     SkPaint* filterPaint(SkPaint* paint);
 
     bool storeDisplayState(DeferredDisplayState& state, int stateDeferFlags);
@@ -283,7 +287,6 @@ public:
     const DrawModifiers& getDrawModifiers() { return mDrawModifiers; }
     void setDrawModifiers(const DrawModifiers& drawModifiers) { mDrawModifiers = drawModifiers; }
 
-    // TODO: what does this mean? no perspective? no rotate?
     ANDROID_API bool isCurrentTransformSimple() {
         return mSnapshot->transform->isSimple();
     }
@@ -325,7 +328,8 @@ public:
     /**
      * Gets the alpha and xfermode out of a paint object. If the paint is null
      * alpha will be 255 and the xfermode will be SRC_OVER. This method does
-     * not multiply the paint's alpha by the current snapshot's alpha.
+     * not multiply the paint's alpha by the current snapshot's alpha, and does
+     * not replace the alpha with the overrideLayerAlpha
      *
      * @param paint The paint to extract values from
      * @param alpha Where to store the resulting alpha
@@ -450,13 +454,21 @@ protected:
 
     /**
      * Gets the alpha and xfermode out of a paint object. If the paint is null
-     * alpha will be 255 and the xfermode will be SRC_OVER.
+     * alpha will be 255 and the xfermode will be SRC_OVER. Accounts for both
+     * snapshot alpha, and overrideLayerAlpha
      *
      * @param paint The paint to extract values from
      * @param alpha Where to store the resulting alpha
      * @param mode Where to store the resulting xfermode
      */
-    inline void getAlphaAndMode(SkPaint* paint, int* alpha, SkXfermode::Mode* mode);
+    inline void getAlphaAndMode(SkPaint* paint, int* alpha, SkXfermode::Mode* mode) const;
+
+    /**
+     * Gets the alpha from a layer, accounting for snapshot alpha and overrideLayerAlpha
+     *
+     * @param layer The layer from which the alpha is extracted
+     */
+    inline float getLayerAlpha(Layer* layer) const;
 
     /**
      * Safely retrieves the mode from the specified xfermode. If the specified