wxMemoryDC docs (Issue #25738)

[PB]

PBfordev created an issue (wxWidgets/wxWidgets#25738)

wxMemoryDC docs has text and code:

Note that the scaling factor of the bitmap determines the scaling factor used by this device context, so when using a memory device context as a back buffer for a window, you should typically create the bitmap using the same scale factor as used by the window, e.g.

void MyWindow::OnPaint(wxPaintEvent&)
{
    wxBitmap bmp;
    bmp.CreateWithDIPSize(GetClientSize(), GetDPIScaleFactor());
    {
        wxMemoryDC memdc(bmp);
        ... use memdc to draw on the bitmap ...
    }
 
    wxPaintDC dc(this);
    dc.DrawBitmap(bmp, wxPoint(0, 0));
}

Is this correct: If my DPI scaling factor is 1.25, the bitmap dimensions will be 1.25 times larger than my window client area (on Windows)?

Docs for wxBitmap::CreateWithDIPSize() has

This should be used when the bitmap size is fixed (e.g. at compile-time) and not if it comes from wxWindow::GetSize() or other similar functions – use CreateWithLogicalSize() in the latter case.

Should not a wxBitmap used as a window backbuffer rather use wxBitmap::CreateWithLogicalSize(), which has in its description:

This should be typically used when creating bitmaps associated to a window area, e.g. to create a bitmap covering the entire window the size parameter should be wxWindow::GetClientSize() and scale should be the wxWindow::GetDPIScaleFactor().

But maybe it is me who is confused and not the docs which is confusing, I am on Windows where the logical and physical pixels are the same...

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[VZ]

vadz left a comment (wxWidgets/wxWidgets#25738)

Thank you, this is indeed wrong and we should use CreateWithLogicalSize() here, will fix.

I am on Windows where the logical and physical pixels are the same...

Yes, but still different from DIP ones, so this is precisely the platform where the current docs are wrong.

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[VZ]

Closed #25738 as completed via a7635a3.

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