Different results on Linux and Windows when GraphicsUnit.Point + transformation matrix are used

[shibaev]

The following C# code produces different results on Linux and Windows - the black rectangle is located at different places:

const float TargetResolution = 300;
double scaleFactor = TargetResolution / 72;
using (Bitmap bitmap = new Bitmap((int)(612 * scaleFactor), (int)(792 * scaleFactor)))
{
    bitmap.SetResolution(TargetResolution, TargetResolution);

    using (System.Drawing.Graphics gr = System.Drawing.Graphics.FromImage(bitmap))
    {
        gr.PageUnit = GraphicsUnit.Point;

        gr.Transform = new Matrix(1, 0, 0, 1, 360, 111);
        gr.FillRectangle(Brushes.Black, 0, 0, 74, 72);
    }

    bitmap.Save("result.png");
}

Linux output:

I tried both libgdiplus 4.2-2 and the latest version compiled with master (dba6fc6) - the result is the same.

Windows output:

[shibaev]

Here is the test for libgdiplus to reproduce the issue:

static void test_issue545()
{
    GpStatus status;
    GpBitmap* bitmap;
    GpGraphics* graphics;
    GpSolidFill* brush;
    CLSID png_clsid = { 0x557cf406, 0x1a04, 0x11d3, { 0x9a, 0x73, 0x0, 0x0, 0xf8, 0x1e, 0xf3, 0x2e } };
    const float TargetResolution = 300;
    double scaleFactor = TargetResolution / 72;
    
    status = GdipCreateBitmapFromScan0((int)(612 * scaleFactor), (int)(792 * scaleFactor), 0, PixelFormat32bppARGB, NULL, &bitmap);
    assertEqualInt(status, Ok);

    status = GdipBitmapSetResolution(bitmap, TargetResolution, TargetResolution);
    assertEqualInt(status, Ok);

    status = GdipGetImageGraphicsContext(bitmap, &graphics);
    assertEqualInt(status, Ok);

    status = GdipSetPageUnit(graphics, UnitPoint);
    assertEqualInt(status, Ok);

    const ARGB FillColor = 0xFF000000;
    status = GdipCreateSolidFill(FillColor, &brush);
    assertEqualInt(status, Ok);

    int rectX = 360;
    int rectY = 111;
    int rectWidth = 74;
    int rectHeight = 72;
#ifndef PASS
    status = GdipTranslateWorldTransform(graphics, rectX, rectY, MatrixOrderPrepend);
    assertEqualInt(status, Ok);

    status = GdipFillRectangleI(graphics, brush, 0, 0, rectWidth, rectHeight);
    assertEqualInt(status, Ok);
#else
    status = GdipFillRectangleI(graphics, brush, rectX, rectY, rectWidth, rectHeight);
    assertEqualInt(status, Ok);
#endif

    int points[] = {
        rectX,
        rectY,
        rectX + rectWidth / 2,
        rectY + rectHeight / 2,
        rectX + rectWidth - 1,
        rectY + rectHeight - 1
    };
    for (int i = 0; i < sizeof(points) / sizeof(points[0]); i += 2)
    {
        ARGB color;
        status = GdipBitmapGetPixel(bitmap, points[i], points[i + 1], &color);
        assertEqualInt(status, Ok);
        assertEqualInt(color, 0);

        status = GdipBitmapGetPixel(bitmap, (int)(points[i] * scaleFactor) + 1, (int)(points[i + 1] * scaleFactor) + 1, &color);
        assertEqualInt(status, Ok);
        assertEqualInt(color, FillColor);
    }

    // Uncomment to visually compare outputs
    // WCHAR* filePath = createWchar("test_issue545.png");
    // status = GdipSaveImageToFile(bitmap, filePath, &png_clsid, NULL);

    GdipDisposeImage((GpImage*)bitmap);
    GdipDeleteGraphics(graphics);
    GdipDeleteBrush(brush);
}

The test will pass if you define PASS symbol or inverse "#ifndef PASS" condition.

[shibaev]

The root cause - GpGraphics::copy_of_ctm is passed to Cairo as is. Instead it should respect page_unit for translation part (x0 and y0).