Convert GDI Plus Bitmap to QPixmap - c++

Question

I have a GdiPlus::Bitmap object, and I need to build a function that converts it to a QPixmap. I'm using Qt 5.8.0, and performance is a concern of mine. I cannot find anything about this online, even searching for GDI+ images to QImages. I don't NEED to access pixel data, so all I have to do is get a GDI+ bitmap converted into something that can be displayed in a QWidget.
The most promising solution I've found so far is to use the fromHBitMap() in QtWin: http://doc.qt.io/qt-5/qtwin.html#fromHBITMAP, but I don't have the required knowledge/experience to be able to understand handles to bitmaps.

Answer 1

I assume you have "Bitmap" object in your hand.
Also I assume your development environment supports both "Windows programming" and "Qt".
Based on above assumptions, you can get the HICON from the "Bitmap" and pass it to "QPixmap QtWin::fromHICON(HICON icon)".
Steps:
First you have to include "Gdiplus.h".
Also include "QtWin".
HICON hIcon;
Status st = <<Your Bitmap Object>>.GetHICON(&hIcon);
QPixmap pixM = QtWin::fromHICON(hIcon);
Try this.
The above code is untested.
It gives an idea.

Answer 2

I don't NEED to access pixel data
You don't, but Qt most certainly does. Qt does not use GDI for rendering widgets - instead, it uses the raster backend that operates on a QImage. Under the hood a QPixmap is just a wrapper around a QImage and it's cheap to convert between the two as it doesn't copy any image data.
So, whatever route you choose will involve QImage at some point, even if it's in the form of a QPixmap.

Answer 3

I had a method of doing this where I flipped the red and blue bytes, then used the following code to convert it from a Gdiplus::Bitmap to a QPixmap:
QPixmap getAsQPixmap(Gdiplus::Bitmap bitmap)
{
// One of my functions to flip blue and red bytes of the image
_flipBlueWithRed();
// m_pBytes is a pointer to an unsigned char that marks the start of the image
// It was retrieved from locking the Gdiplus::Bitmap
auto result = QPixmap::fromImage(QImage(m_pBytes, getWidth(), getHeight(), QImage::Format_RGB888));
// Restore data back to original state
_flipBlueWithRed();
return result;
}
This method, however, was slow and took a good 60 milliseconds to execute. So my new method was to save the Gdiplus::Bitmap to file, and use the path to that file to read in from QPixmap's constructor. This method is MUCH faster, around 5 milliseconds.
QPixmap getAsQPixmap(GdiPlus::Bitmap bitmap)
{
std::string path = "C:/path_to_img.....png";
// One of my functions to unlock the Gdi+ bitmap
_unlockGdiPlusBitmap();
// Get Clsid
CLSID pngClsid;
getEncoderClsid(format_mime, &pngClsid);
// Save bitmap
// stringToWString() was a function that I wrote to convert a standard string to be a wide string
bitmap->Save(stringToWString(path).c_str(), static_cast<const CLSID*>(&pngClsid));
// Lock bitmap
_lockGdiPlusBitmap();
// Create the QPixmap
QPixmap new_img(path);
return new_img;
}