[PATCH v2 2/2] gdiplus: GdipGraphicsClear() should overwrite pixels, not alpha blend

[Damjan Jovanovic]

All sample code in the Python Pyglet library suffers from terrible
text corruption, caused by the inability to erase the background
between rendering sequential font glyphs, resulting in leftovers
from previous letters mixing with the image of new letters.

This is because it attempts to erase the background by calling
GdipGraphicsClear() with ARGB color 0x00000000 (completely
transparent black), and in our gdiplus alpha blending that into
the background has no effect. It should be using
CompositeModeSourceCopy to overwrite the background with that brush
instead.

Try 2 split the patch.

Wine-Bug: https://bugs.winehq.org/show_bug.cgi?id=30506

Signed-off-by: Damjan Jovanovic <damjan.jov at gmail.com>
---
 dlls/gdiplus/graphics.c    |  4 ++++
 dlls/gdiplus/tests/image.c | 43 ++++++++++++++++++++++++++++++++++++++
 2 files changed, 47 insertions(+)
-------------- next part --------------
diff --git a/dlls/gdiplus/graphics.c b/dlls/gdiplus/graphics.c
index e0a013896f..e485e5c99f 100644
--- a/dlls/gdiplus/graphics.c
+++ b/dlls/gdiplus/graphics.c
@@ -4949,6 +4949,7 @@ GpStatus WINGDIPAPI GdipGraphicsClear(GpGraphics *graphics, ARGB color)
     GpSolidFill *brush;
     GpStatus stat;
     GpRectF wnd_rect;
+    CompositingMode prev_comp_mode;
 
     TRACE("(%p, %x)\n", graphics, color);
 
@@ -4969,8 +4970,11 @@ GpStatus WINGDIPAPI GdipGraphicsClear(GpGraphics *graphics, ARGB color)
         return stat;
     }
 
+    GdipGetCompositingMode(graphics, &prev_comp_mode);
+    GdipSetCompositingMode(graphics, CompositingModeSourceCopy);
     GdipFillRectangle(graphics, (GpBrush*)brush, wnd_rect.X, wnd_rect.Y,
                                                  wnd_rect.Width, wnd_rect.Height);
+    GdipSetCompositingMode(graphics, prev_comp_mode);
 
     GdipDeleteBrush((GpBrush*)brush);
 
diff --git a/dlls/gdiplus/tests/image.c b/dlls/gdiplus/tests/image.c
index 74c471e9ac..62cd79fec6 100644
--- a/dlls/gdiplus/tests/image.c
+++ b/dlls/gdiplus/tests/image.c
@@ -5485,6 +5485,48 @@ todo_wine
     HeapFree(GetProcessHeap(), 0, data);
 }
 
+static void test_graphics_clear(void)
+{
+    BYTE argb[8] = { 0x11,0x22,0x33,0x80, 0xff,0xff,0xff,0 };
+    BYTE cleared[8] = { 0,0,0,0, 0,0,0,0 };
+    BYTE *bits;
+    GpBitmap *bitmap;
+    GpGraphics *graphics;
+    BitmapData data;
+    GpStatus status;
+    int match;
+
+    status = GdipCreateBitmapFromScan0(2, 1, 8, PixelFormat32bppARGB, argb, &bitmap);
+    expect(Ok, status);
+
+    status = GdipGetImageGraphicsContext((GpImage*)bitmap, &graphics);
+    expect(Ok, status);
+
+    status = GdipGraphicsClear(graphics, 0x00000000);
+    expect(Ok, status);
+
+    status = GdipBitmapLockBits(bitmap, NULL, ImageLockModeRead, PixelFormat32bppARGB, &data);
+    expect(Ok, status);
+    ok(data.Width == 2, "expected 2, got %d\n", data.Width);
+    ok(data.Height == 1, "expected 1, got %d\n", data.Height);
+    ok(data.Stride == 8, "expected 8, got %d\n", data.Stride);
+    ok(data.PixelFormat == PixelFormat32bppARGB, "expected PixelFormat32bppARGB, got %d\n", data.PixelFormat);
+    match = !memcmp(data.Scan0, cleared, sizeof(cleared));
+    ok(match, "bits don't match\n");
+    if (!match)
+    {
+        bits = data.Scan0;
+        trace("format %#x, bits %02x,%02x,%02x,%02x %02x,%02x,%02x,%02x\n", PixelFormat32bppPARGB,
+               bits[0], bits[1], bits[2], bits[3], bits[4], bits[5], bits[6], bits[7]);
+    }
+    status = GdipBitmapUnlockBits(bitmap, &data);
+    expect(Ok, status);
+
+    status = GdipDeleteGraphics(graphics);
+    expect(Ok, status);
+    GdipDisposeImage((GpImage *)bitmap);
+}
+
 START_TEST(image)
 {
     HMODULE mod = GetModuleHandleA("gdiplus.dll");
@@ -5560,6 +5602,7 @@ START_TEST(image)
     test_histogram();
     test_imageabort();
     test_GdipLoadImageFromStream();
+    test_graphics_clear();
 
     GdiplusShutdown(gdiplusToken);
 }