Improved BMP decoding.

The BMP decoder did not handle images that are not stored upside down but straight up (in which case the height is negative). Also with RLE4 or RLE8 compressed images the 'end of scanline' RLE marker was not handled correctly. Fixed the issues and added a unit test for them.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@68766 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/common/imagbmp.cpp

@@ -533,6 +533,14 @@ bool wxBMPHandler::DoLoadDib(wxImage * image, int width, int height,
 
     wxON_BLOCK_EXIT1(&BMPPalette::Free, cmap);
 
+    bool isUpsideDown = true;
+
+    if (height < 0)
+    {
+        isUpsideDown = false;
+        height = -height;
+    }
+
     // destroy existing here instead of:
     image->Destroy();
     image->Create(width, height);
@@ -702,9 +710,10 @@ bool wxBMPHandler::DoLoadDib(wxImage * image, int width, int height,
     // this case (see #10915)
     bool hasValidAlpha = false;
 
-    /* BMPs are stored upside down */
-    for ( int line = (height - 1); line >= 0; line-- )
+    for ( int row = 0; row < height; row++ )
     {
+        int line = isUpsideDown ? height - 1 - row : row;
+
         int linepos = 0;
         for ( int column = 0; column < width ; )
         {
@@ -734,21 +743,24 @@ bool wxBMPHandler::DoLoadDib(wxImage * image, int width, int height,
                         {
                             if ( aByte == 0 )
                             {
-                                if ( column > 0 )
-                                    column = width;
+                                // end of scanline marker
+                                column = width;
+                                row--;
                             }
                             else if ( aByte == 1 )
                             {
+                                // end of RLE data marker, stop decoding
                                 column = width;
-                                line = -1;
+                                row = height;
                             }
                             else if ( aByte == 2 )
                             {
+                                // delta marker, move in image
                                 aByte = stream.GetC();
                                 column += aByte;
                                 linepos = column * bpp / 4;
                                 aByte = stream.GetC();
-                                line -= aByte; // upside down
+                                row += aByte; // upside down
                             }
                             else
                             {
@@ -817,20 +829,24 @@ bool wxBMPHandler::DoLoadDib(wxImage * image, int width, int height,
                         {
                             if ( aByte == 0 )
                             {
-                                /* column = width; */
+                                // end of scanline marker
+                                column = width;
+                                row--;
                             }
                             else if ( aByte == 1 )
                             {
+                                // end of RLE data marker, stop decoding
                                 column = width;
-                                line = -1;
+                                row = height;
                             }
                             else if ( aByte == 2 )
                             {
+                                // delta marker, move in image
                                 aByte = stream.GetC();
                                 column += aByte;
                                 linepos = column * bpp / 8;
                                 aByte = stream.GetC();
-                                line += aByte;
+                                row -= aByte;
                             }
                             else
                             {

tests/image/image.cpp

@@ -77,6 +77,7 @@ private:
         CPPUNIT_TEST( ReadCorruptedTGA );
         CPPUNIT_TEST( GIFComment );
         CPPUNIT_TEST( DibPadding );
+        CPPUNIT_TEST( BMPFlippingAndRLECompression );
     CPPUNIT_TEST_SUITE_END();
 
     void LoadFromSocketStream();
@@ -90,6 +91,7 @@ private:
     void ReadCorruptedTGA();
     void GIFComment();
     void DibPadding();
+    void BMPFlippingAndRLECompression();
 
     DECLARE_NO_COPY_CLASS(ImageTestCase)
 };
@@ -1262,6 +1264,26 @@ void ImageTestCase::DibPadding()
     CPPUNIT_ASSERT( image.SaveFile(memOut, wxBITMAP_TYPE_ICO) );
 }
 
+static void CompareBMPImage(const wxString& file1, const wxString& file2)
+{
+    wxImage image1(file1);
+    CPPUNIT_ASSERT( image1.IsOk() );
+
+    wxImage image2(file2);
+    CPPUNIT_ASSERT( image2.IsOk() );
+
+    CompareImage(*wxImage::FindHandler(wxBITMAP_TYPE_BMP), image1, 0, &image2);
+}
+
+void ImageTestCase::BMPFlippingAndRLECompression()
+{
+    CompareBMPImage("image/horse_grey.bmp", "image/horse_grey_flipped.bmp");
+
+    CompareBMPImage("image/horse_rle8.bmp", "image/horse_grey.bmp");
+    CompareBMPImage("image/horse_rle8.bmp", "image/horse_rle8_flipped.bmp");
+
+    CompareBMPImage("image/horse_rle4.bmp", "image/horse_rle4_flipped.bmp");
+}
 #endif //wxUSE_IMAGE