Taking Screenshots with wxWidgets under Mac OS is Really Tricky.

Taking screenshots is a very common task and it was a must for one of my current projects. What was a surprise when I understood that my favourite toolkit can’t do that in cross-platform manner.

It is official bug that wxScreenDC does not work properly under Mac OS and you can’t use Blit() message for copying screen onto wxMemoryDC.

After digging the Internet I found a kind of solution which used OpenGL and created wxWidgets-based class which takes screenshots also under Mac OS. It was really hard task for me because I haven’t used neither Carbon nor Cocoa before. However everything works now and I’m happy.

Here it is:

wxScreenshotMaker.h

#pragma once

#include <wx/wx.h>
#ifdef __WXMAC__
#include <OpenGL/OpenGL.h>
#endif

class wxScreenshotMaker
{
public:
 wxScreenshotMaker();
 ~wxScreenshotMaker();
 wxBitmap GetScreenshot();
private:
#ifdef __WXMAC__

 int screenWidth;
 int screenHeight;

 CGLContextObj glContextObj;
 int rowSize;
 unsigned char * glBitmapData;
 unsigned char * glRowData;

 void InitOpenGL();
 void GrabGLScreen();
 void SwizzleBitmap();
 void FinalizeOpenGL();
#endif
};

wxScreenshotMaker.cpp

#include "wxScreenshotMaker.h"
#ifndef __WXMAC__
#include <wx/dcscreen.h>
#else
#include <CoreFoundation/CoreFoundation.h>
#include <ApplicationServices/ApplicationServices.h>
#include <OpenGL/gl.h>
#endif

wxScreenshotMaker::wxScreenshotMaker()
#ifdef __WXMAC__
: screenWidth(0), screenHeight(0), glBitmapData(NULL), glRowData(NULL)
#endif
{
#ifdef __WXMAC__
 wxSize size = wxGetDisplaySize();
 screenWidth = size.GetWidth();
 screenHeight = size.GetHeight();
 InitOpenGL();
#endif
}

wxScreenshotMaker::~wxScreenshotMaker()
{
#ifdef __WXMAC__
 FinalizeOpenGL();
#endif
}

wxBitmap wxScreenshotMaker::GetScreenshot()
{
#ifndef __WXMAC__
 wxScreenDC screenDC;
 wxBitmap bmp(screenDC.GetSize().GetWidth(), screenDC.GetSize().GetHeight());
 wxMemoryDC mdc(bmp);
 mdc.Blit(0, 0, bmp.GetWidth(), bmp.GetHeight(), &screenDC, 0, 0);
 mdc.SelectObject(wxNullBitmap);
 return bmp;
#else
 if(glBitmapData)
 {
  GrabGLScreen();
  wxImage img = wxImage(screenWidth, screenHeight, glBitmapData, true);
  return wxBitmap(img.Copy());
 }
#endif
}

#ifdef __WXMAC__

void wxScreenshotMaker::InitOpenGL()
{
 do
 {
  rowSize = screenWidth * 3;
  rowSize = (rowSize + 2) & ~2;

  glRowData = (unsigned char *)realloc(glRowData, rowSize);
  glBitmapData = (unsigned char *)realloc(glBitmapData, rowSize * screenHeight);

  bzero(glRowData, rowSize);
  bzero(glBitmapData, rowSize * screenHeight);

  CGDirectDisplayID display = CGMainDisplayID();
  CGLPixelFormatObj pixelFormatObj ;
  GLint numPixelFormats;
  CGLPixelFormatAttribute attribs[] =
  {
   kCGLPFAFullScreen,
   kCGLPFADisplayMask,
   (CGLPixelFormatAttribute)0,    /* Display mask bit goes here */   (CGLPixelFormatAttribute)0
  };

  attribs[2] = (CGLPixelFormatAttribute) CGDisplayIDToOpenGLDisplayMask(display);

  /* Build a full-screen GL context */  CGLChoosePixelFormat( attribs, &pixelFormatObj, &numPixelFormats );
  if ( pixelFormatObj == NULL ) break;
  CGLCreateContext( pixelFormatObj, NULL, &glContextObj ) ;
  CGLDestroyPixelFormat( pixelFormatObj ) ;
  if ( glContextObj == NULL ) break;
  CGLSetCurrentContext( glContextObj ) ;
  CGLSetFullScreen( glContextObj ) ;
 }
 while(false);
}

void wxScreenshotMaker::FinalizeOpenGL()
{
    CGLSetCurrentContext( NULL );
    CGLClearDrawable( glContextObj );
    CGLDestroyContext( glContextObj );
 free(glRowData);
 free(glBitmapData);
}

void wxScreenshotMaker::GrabGLScreen()
{
 glReadBuffer(GL_FRONT);
    /* Read framebuffer into our bitmap */    glFinish();
    glPixelStorei(GL_PACK_ALIGNMENT, 3);
    glPixelStorei(GL_PACK_ROW_LENGTH, 0);
    glPixelStorei(GL_PACK_SKIP_ROWS, 0);
    glPixelStorei(GL_PACK_SKIP_PIXELS, 0);
    /*
      * Fetch the data in RGB format, matching the bitmap context.
      */    glReadPixels((GLint)0, (GLint)0, (GLint)screenWidth, (GLint)screenHeight,
  GL_RGB, GL_BYTE, glBitmapData);

 SwizzleBitmap();
}

void wxScreenshotMaker::SwizzleBitmap()
{
 int top, bottom;
 top = 0;
    bottom = screenHeight - 1;
 void * base = glBitmapData;
 void * topP = NULL;
 void * bottomP = NULL;

 while(top < bottom)
 {
  topP = (void *)((top * rowSize) + (intptr_t)base);
        bottomP = (void *)((bottom * rowSize) + (intptr_t)base);

  bcopy( topP, glRowData, rowSize );
        bcopy( bottomP, topP, rowSize );
        bcopy( glRowData, bottomP, rowSize );

        ++top;
        --bottom;
 }
}

#endif

Sample

wxScreenshotMaker screenshot;
m_Canvas->SetBitmap(screenshot.GetScreenshot());
m_Canvas->Refresh();

Download wxScreenshotMaker and sample project.