wxMSW: transparency bug in wxMemoryDC ?

alchemist · Post by **alchemist** » Tue Apr 29, 2008 1:58 pm

Hello,

I want to programmatically create transparent bitmaps based on transparent bitmaps (from PNG files) and DC methods (e.g. DrawRectangle), and later draw them transparently on the window's DC.

To achieve that, I use wxMemoryDC, but it seems that the resulting bitmap is completely screwed up: it is as if the alpha channel is lost, and the whole bitmap is completely transparent, event on the opaque elements.

On my example, I draw two filled red rectangles:
- one from a transparent PNG file (transparent.png)
- one with wxDC.DrawRectangle

This operation is first done directly on the window filled with a ugly coloured gradient, then into a wxBitmap which is drawn and blitted. The bitmap is also saved as a png file "result.png".

In MacOS, the resulting file is correct (2 red rectangles on a transparent background), but not in Windows (a fully transparent bitmap with maybe 2 reds transparent rectangles).

And on the display, this is also not correct.

Is there a bug in wxMSW ?

Code: Select all

#include "wx/wx.h"

class MyApp : public wxApp
{
public:
    virtual bool OnInit();
};

class MyFrame : public wxFrame
{
public:
    MyFrame(const wxString& title);
private:
  void OnPaint(wxPaintEvent &event);
  void OnErase(wxEraseEvent &event);

  void drawStuff(wxDC *dc);
  
  wxBitmap file_;

  DECLARE_EVENT_TABLE()
};

IMPLEMENT_APP(MyApp)

bool MyApp::OnInit()
{
    if ( !wxApp::OnInit() )
        return false;
	wxImage::AddHandler(new wxPNGHandler);

    MyFrame *frame = new MyFrame(_T("Transparency tests"));
    frame->Show(true);
    return true;
}

BEGIN_EVENT_TABLE(MyFrame, wxFrame)
  EVT_PAINT(MyFrame::OnPaint)
  EVT_ERASE_BACKGROUND(MyFrame::OnErase)
END_EVENT_TABLE()


MyFrame::MyFrame(const wxString& title)
       : wxFrame(NULL, wxID_ANY, title, wxDefaultPosition, wxDefaultSize, wxSYSTEM_MENU | wxCAPTION | wxCLOSE_BOX | wxCLIP_CHILDREN)
{
	file_ = wxBitmap(wxImage(L"transparent.png"));
	SetClientSize(wxSize(340, 120));
}


void MyFrame::OnErase(wxEraseEvent &event)
{
	wxDC *dc(event.GetDC());
	dc->GradientFillLinear(wxRect(GetClientRect()), *wxBLUE, *wxGREEN, wxSOUTH);
}



void MyFrame::drawStuff(wxDC *dc)
{
	// draw a red rectangle 
	dc->SetBrush(*wxRED_BRUSH);
	dc->SetPen(*wxTRANSPARENT_PEN);
	dc->DrawRectangle(10, 10, 30, 30);

	// draw the same rectangle from a file
	dc->DrawBitmap(file_, 50, 00, true);
}

void MyFrame::OnPaint(wxPaintEvent& WXUNUSED(event))
{
	wxPaintDC dc(this);

	drawStuff(&dc);
	
	wxBitmap buf(100, 50);
	buf.UseAlpha();
	
	wxMemoryDC *memDC = new wxMemoryDC(buf);
	memDC->SetBackground(*wxTRANSPARENT_BRUSH);
	memDC->Clear();
	delete memDC;

	memDC = new wxMemoryDC(buf);
	drawStuff(memDC);
	delete memDC;
	
	buf.SaveFile(_("result.png"), wxBITMAP_TYPE_PNG);

	// DrawBitmap
	dc.DrawBitmap(buf, 120,  0, true);
	dc.DrawBitmap(buf, 120, 70, false);
	
	// blit
	memDC = new wxMemoryDC(buf);
	dc.Blit(240,  0, 100, 50, memDC, 0, 0);
	dc.Blit(240, 70, 100, 50, memDC, 0, 0, wxCOPY, true);
	delete memDC;
}