turn dithering off if it's not needed

libs/surfaceflinger/Layer.cpp

@@ -51,7 +51,8 @@ Layer::Layer(SurfaceFlinger* flinger, DisplayID display,
         const sp<Client>& c, int32_t i)
     :   LayerBaseClient(flinger, display, c, i),
         mSecure(false),
-        mNeedsBlending(true)
+        mNeedsBlending(true),
+        mNeedsDithering(false)
 {
     // no OpenGL operation is possible here, since we might not be
     // in the OpenGL thread.
@@ -106,10 +107,16 @@ status_t Layer::ditch()
 status_t Layer::setBuffers( uint32_t w, uint32_t h,
                             PixelFormat format, uint32_t flags)
 {
+    // this surfaces pixel format
     PixelFormatInfo info;
     status_t err = getPixelFormatInfo(format, &info);
     if (err) return err;
 
+    // the display's pixel format
+    const DisplayHardware& hw(graphicPlane(0).displayHardware());
+    PixelFormatInfo displayInfo;
+    getPixelFormatInfo(hw.getFormat(), &displayInfo);
+
     uint32_t bufferFlags = 0;
     if (flags & ISurfaceComposer::eSecure)
         bufferFlags |= Buffer::SECURE;
@@ -119,6 +126,12 @@ status_t Layer::setBuffers( uint32_t w, uint32_t h,
     mHeight = h;
     mSecure = (bufferFlags & Buffer::SECURE) ? true : false;
     mNeedsBlending = (info.h_alpha - info.l_alpha) > 0;
+
+    // we use the red index
+    int displayRedSize = displayInfo.getSize(PixelFormatInfo::INDEX_RED);
+    int layerRedsize = info.getSize(PixelFormatInfo::INDEX_RED);
+    mNeedsDithering = layerRedsize > displayRedSize;
+
     mBufferFlags = bufferFlags;
     for (size_t i=0 ; i<NUM_BUFFERS ; i++) {
         mBuffers[i] = new Buffer();

libs/surfaceflinger/Layer.h

@@ -68,6 +68,7 @@ public:
     virtual void unlockPageFlip(const Transform& planeTransform, Region& outDirtyRegion);
     virtual void finishPageFlip();
     virtual bool needsBlending() const      { return mNeedsBlending; }
+    virtual bool needsDithering() const     { return mNeedsDithering; }
     virtual bool isSecure() const           { return mSecure; }
     virtual sp<Surface> createSurface() const;
     virtual status_t ditch();
@@ -109,6 +110,7 @@ private:
             bool            mSecure;
             int32_t         mFrontBufferIndex;
             bool            mNeedsBlending;
+            bool            mNeedsDithering;
             Region          mPostedDirtyRegion;
             sp<FreezeLock>  mFreezeLock;
             PixelFormat     mFormat;

libs/surfaceflinger/LayerBase.cpp

@@ -393,14 +393,6 @@ void LayerBase::drawWithOpenGL(const Region& clip, const Texture& texture) const
     
     glEnable(GL_TEXTURE_2D);
 
-    // Dithering...
-    bool fast = !(mFlags & DisplayHardware::SLOW_CONFIG);
-    if (fast || s.flags & ISurfaceComposer::eLayerDither) {
-        glEnable(GL_DITHER);
-    } else {
-        glDisable(GL_DITHER);
-    }
-
     if (UNLIKELY(s.alpha < 0xFF)) {
         // We have an alpha-modulation. We need to modulate all
         // texture components by alpha because we're always using 
@@ -516,6 +508,12 @@ void LayerBase::validateTexture(GLint textureName) const
         glTexParameterx(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
         glTexParameterx(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);
     }
+
+    if (needsDithering()) {
+        glEnable(GL_DITHER);
+    } else {
+        glDisable(GL_DITHER);
+    }
 }
 
 void LayerBase::loadTexture(Texture* texture, GLint textureName, 

libs/surfaceflinger/LayerBase.h

@@ -187,6 +187,11 @@ public:
      */
     virtual bool needsBlending() const  { return false; }
 
+    /**
+     * needsDithering - true if this surface needs dithering
+     */
+    virtual bool needsDithering() const { return false; }
+
     /**
      * transformed -- true is this surface needs a to be transformed
      */

libs/surfaceflinger/SurfaceFlinger.cpp

@@ -1497,11 +1497,12 @@ status_t SurfaceFlinger::dump(int fd, const Vector<String16>& args)
                     "+ %s %p\n"
                     "      "
                     "z=%9d, pos=(%4d,%4d), size=(%4d,%4d), "
-                    "needsBlending=%1d, invalidate=%1d, "
+                    "needsBlending=%1d, needsDithering=%1d, invalidate=%1d, "
                     "alpha=0x%02x, flags=0x%08x, tr=[%.2f, %.2f][%.2f, %.2f]\n",
                     layer->getTypeID(), layer.get(),
                     s.z, layer->tx(), layer->ty(), s.w, s.h,
-                    layer->needsBlending(), layer->contentDirty,
+                    layer->needsBlending(), layer->needsDithering(),
+                    layer->contentDirty,
                     s.alpha, s.flags,
                     s.transform[0], s.transform[1],
                     s.transform[2], s.transform[3]);