Damjan Jovanovic : gdiplus: GdipGraphicsClear() should overwrite pixels, not alpha blend.

[Alexandre Julliard]

Module: wine
Branch: master
Commit: 3c8784bf9c1a897de021587437c5ec4f648c84a1
URL:    https://source.winehq.org/git/wine.git/?a=commit;h=3c8784bf9c1a897de021587437c5ec4f648c84a1

Author: Damjan Jovanovic <damjan.jov at gmail.com>
Date:   Fri Dec 13 05:17:54 2019 +0200

gdiplus: GdipGraphicsClear() should overwrite pixels, not alpha blend.

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.

Wine-Bug: https://bugs.winehq.org/show_bug.cgi?id=30506
Signed-off-by: Damjan Jovanovic <damjan.jov at gmail.com>
Signed-off-by: Vincent Povirk <vincent at codeweavers.com>
Signed-off-by: Alexandre Julliard <julliard at winehq.org>

---

 dlls/gdiplus/graphics.c    |  4 ++++
 dlls/gdiplus/tests/image.c | 43 +++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 47 insertions(+)

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);
 }