Added a new function to know if an image is using an alpha channel.

This all will be useful for my next patch: Repair windows opacity break in SDL mode.

2 files changed, 49 insertions, 3 deletions

diff --git a/src/resources/image.cpp b/src/resources/image.cpp
index b3f7140d..0540d543 100644
--- a/src/resources/image.cpp
+++ b/src/resources/image.cpp
@@ -48,6 +48,8 @@ Image::Image(SDL_Surface *image):
     {
         mBounds.w = mSDLSurface->w;
         mBounds.h = mSDLSurface->h;
+
+        mAlphaChannel = hasAlphaChannel();
         mLoaded = true;
     }
     else
@@ -64,7 +66,8 @@ Image::Image(GLuint glimage, int width, int height, int texWidth, int texHeight)
     mTexWidth(texWidth),
     mTexHeight(texHeight),
     mSDLSurface(0),
-    mAlpha(1.0)
+    mAlpha(1.0),
+    mAlphaChannel(true)
 {
     mBounds.x = 0;
     mBounds.y = 0;
@@ -189,6 +192,43 @@ bool Image::isAnOpenGLOne() const
 #endif
 }
 
+bool Image::hasAlphaChannel()
+{
+    if (mLoaded)
+        return mAlphaChannel;
+
+#ifdef USE_OPENGL
+    if (mUseOpenGL)
+        return true;
+#endif
+
+    if (!mSDLSurface)
+        return false;
+
+    bool hasAlpha = false;
+
+    if (mSDLSurface->format->BitsPerPixel == 32)
+    {
+        // Figure out whether the image uses its alpha layer
+        for (int i = 0; i < mSDLSurface->w * mSDLSurface->h; ++i)
+        {
+            Uint8 r, g, b, a;
+            SDL_GetRGBA(
+                    ((Uint32*) mSDLSurface->pixels)[i],
+                    mSDLSurface->format,
+                    &r, &g, &b, &a);
+
+            if (a != 255)
+            {
+                hasAlpha = true;
+                break;
+            }
+        }
+    }
+
+    return hasAlpha;
+}
+
 void Image::setAlpha(float a)
 {
     if (mAlpha == a)
diff --git a/src/resources/image.h b/src/resources/image.h
index c97523a9..535a3536 100644
--- a/src/resources/image.h
+++ b/src/resources/image.h
@@ -111,12 +111,18 @@ class Image : public Resource
         { return mBounds.h; }
 
         /**
-         * Tells if the image was loade using OpenGL or SDL
+         * Tells if the image was loaded using OpenGL or SDL
          * @return true if OpenGL, false if SDL.
          */
         bool isAnOpenGLOne() const;
 
         /**
+         * Tells if the image has got an alpha channel
+         * @return true if OpenGL, false if SDL.
+         */
+        bool hasAlphaChannel();
+
+        /**
          * Sets the alpha value of this image.
          */
         virtual void setAlpha(float alpha);
@@ -181,6 +187,7 @@ class Image : public Resource
         SDL_Rect mBounds;
         bool mLoaded;
         float mAlpha;
+        bool mAlphaChannel;
 
       // -----------------------
       // SDL protected members
@@ -191,7 +198,6 @@ class Image : public Resource
 
         static Image *_SDLload(SDL_Surface *tmpImage);
 
-
         SDL_Surface *mSDLSurface;