glReadPixels works fine in Linux but not in Windows 10

DavidRS

I'm trying to save an image drawn in an QOpenGLWindow. The code is simple

void XYScene::CreateTIFF()
{
   emit AlertMsg("Creating Screenshot image as TIFF ....",' ');

   int nNoRGBABytes = XImageSize*YImageSize*4;

   GLubyte* ScreenData=nullptr;
   try{
       ScreenData = new GLubyte[size_t(nNoRGBABytes)];
   }
   catch (std::bad_alloc& ba)
   {
       emit AlertMsg(QString("Unable to allocate memory for TIFF creation<br>Reason: %1").arg(ba.what()),'r');
       return;
   }
   glPixelStorei(GL_PACK_ALIGNMENT, 1);
   glReadPixels(0, 0, XImageSize, YImageSize, GL_RGBA, GL_UNSIGNED_BYTE, ScreenData);
  .........

This code works perfectly in Linux (Fedora using XWindows and g++) but not in Windows (Win10 using MSVC 2019). When I look at ScreenData in Windows it's filled with zero's which would seem to indicate that something is being transferred but not from the right place. Is there some code that I need to insert for it to work on Windows?

This is on a dual-boot machine so that all that differs is the Operating system and the NVIDIA drivers (and whatever difference there is in how OpenGL is implemented). In Windows Qt::AA_UseDesktopOpenGL is set via QCoreApplication.

DavidRS

I did, but same problem - it turns out that in Windows, with my setup, the image is on the back buffer, not the front, so the solution was:

void XYScene::CreateTIFF()
{
   emit AlertMsg("Creating Screenshot image as TIFF ....",' ');

   int nNoRGBABytes = XImageSize*YImageSize*4;

   GLubyte* ScreenData=nullptr;
   try{
       ScreenData = new GLubyte[size_t(nNoRGBABytes)];
   }
   catch (std::bad_alloc& ba)
   {
       emit AlertMsg(QString("Unable to allocate memory for TIFF creation<br>Reason: %1").arg(ba.what()),'r');
       return;
   }
#ifdef WIN32
   QOpenGLContext* ctx = context();
   QSurface* ctsurf = ctx->surface();
   ctx->swapBuffers(ctsurf);       // swap buffers to bring image to the front buffer to be read.
#endif
   glReadPixels(0, 0, XImageSize, YImageSize, GL_RGBA, GL_UNSIGNED_BYTE, ScreenData);
#ifdef WIN32
   ctx->swapBuffers(ctsurf);  // need this to see the image otherwise screen is black 
   makeCurrent();
#endif

Why this happening in my setup and whether it would occur on other Window systems, I don't know.

SGaist

Hi,

Why not use QOpenGLWindow::grabFramebuffer ?

DavidRS

I did, but same problem - it turns out that in Windows, with my setup, the image is on the back buffer, not the front, so the solution was:

void XYScene::CreateTIFF()
{
   emit AlertMsg("Creating Screenshot image as TIFF ....",' ');

   int nNoRGBABytes = XImageSize*YImageSize*4;

   GLubyte* ScreenData=nullptr;
   try{
       ScreenData = new GLubyte[size_t(nNoRGBABytes)];
   }
   catch (std::bad_alloc& ba)
   {
       emit AlertMsg(QString("Unable to allocate memory for TIFF creation<br>Reason: %1").arg(ba.what()),'r');
       return;
   }
#ifdef WIN32
   QOpenGLContext* ctx = context();
   QSurface* ctsurf = ctx->surface();
   ctx->swapBuffers(ctsurf);       // swap buffers to bring image to the front buffer to be read.
#endif
   glReadPixels(0, 0, XImageSize, YImageSize, GL_RGBA, GL_UNSIGNED_BYTE, ScreenData);
#ifdef WIN32
   ctx->swapBuffers(ctsurf);  // need this to see the image otherwise screen is black 
   makeCurrent();
#endif

Why this happening in my setup and whether it would occur on other Window systems, I don't know.