Add alpha blending for wxImage::Paste

Add test cases for wxImage::Paste.

Closes #12458.

Co-Authored-By: Rachel Mark <kramdar@gmail.com>

tests/testimage.h

@@ -30,8 +30,9 @@ namespace Catch
 class ImageRGBMatcher : public Catch::MatcherBase<wxImage>
 {
 public:
-    ImageRGBMatcher(const wxImage& image)
+    ImageRGBMatcher(const wxImage& image, int tolerance)
         : m_image(image)
+        , m_tolerance(tolerance)
     {
     }
 
@@ -53,7 +54,8 @@ public:
         {
             for ( int y = 0; y < m_image.GetHeight(); ++y )
             {
-                if ( *d1 != *d2 )
+                const unsigned char diff = *d1 > * d2 ? *d1 - *d2 : *d2 - *d1;
+                if (diff > m_tolerance)
                 {
                     m_diffDesc.Printf
                                (
@@ -72,11 +74,11 @@ public:
             }
         }
 
-        // We should never get here as we know that the images are different
-        // and so should have returned from inside the loop above.
-        wxFAIL_MSG("unreachable");
+        // We can only get here when the images are different AND we've not exited the
+        // method from the loop. That implies the tolerance must have caused this.
+        wxASSERT_MSG(m_tolerance > 0, "Unreachable without tolerance");
 
-        return false;
+        return true;
     }
 
     std::string describe() const wxOVERRIDE
@@ -92,12 +94,67 @@ public:
 
 private:
     const wxImage m_image;
+    const int m_tolerance;
     mutable wxString m_diffDesc;
 };
 
 inline ImageRGBMatcher RGBSameAs(const wxImage& image)
 {
-    return ImageRGBMatcher(image);
+    return ImageRGBMatcher(image, 0);
+}
+
+// Allows small differences (within given tolerance) for r, g, and b values.
+inline ImageRGBMatcher RGBSimilarTo(const wxImage& image, int tolerance)
+{
+    return ImageRGBMatcher(image, tolerance);
+}
+
+class ImageAlphaMatcher : public Catch::MatcherBase<wxImage>
+{
+public:
+    ImageAlphaMatcher(unsigned char alpha)
+        : m_alpha(alpha)
+    {
+    }
+
+    bool match(const wxImage& other) const wxOVERRIDE
+    {
+        if (!other.HasAlpha())
+        {
+            m_diffDesc = "no alpha data";
+            return false;
+        }
+
+        unsigned char center_alpha =
+            *(other.GetAlpha() + (other.GetWidth() / 2) + (other.GetHeight() / 2 * other.GetWidth()));
+
+        if (m_alpha != center_alpha)
+        {
+            m_diffDesc.Printf("got alpha %u", center_alpha);
+            return false;
+        }
+
+        return true;
+    }
+
+    std::string describe() const wxOVERRIDE
+    {
+        std::string desc;
+
+        if (!m_diffDesc.empty())
+            desc = m_diffDesc.ToStdString(wxConvUTF8);
+
+        return desc;
+    }
+
+private:
+    const unsigned char m_alpha;
+    mutable wxString m_diffDesc;
+};
+
+inline ImageAlphaMatcher CenterAlphaPixelEquals(unsigned char alpha)
+{
+    return ImageAlphaMatcher(alpha);
 }
 
 #endif // _WX_TESTS_TESTIMAGE_H_