How to use GDI+ library to decode a jpeg in memory?

Question

GDI+ provides a Image class, and you can use this class to read a image file with one format and then save this file to another format. But if I want to just decode a jpeg file (already loaded into memory), how can I do it?

Answer 1

You can use SHCreateMemStream and Gdiplus::Image::FromStream

#include <Window.h>
#include <Gdiplus.h>
#include <Shlwapi.h>
#include <atlbase.h>
...
CComPtr<IStream> stream;
stream.Attach(SHCreateMemStream(buf, bufsize));
Gdiplus::Image *image = Gdiplus::Image::FromStream(stream);

Where buf contains jpeg data (or any other compatible image format) and bufsize is the length of that data.

SHCreateMemStream needs "Shlwapi.lib" library.

Example:

void foo(HDC hdc)
{
    //Read jpeg from input file in to buf:
    HANDLE hfile = CreateFile(L"test.jpg", 
        GENERIC_READ, 0, NULL, OPEN_ALWAYS, FILE_ATTRIBUTE_NORMAL, NULL);
    if (!hfile) return;

    DWORD bufsize = GetFileSize(hfile, NULL);
    BYTE *buf = new BYTE[bufsize];

    DWORD temp;
    ReadFile(hfile, buf, bufsize, &temp, 0);

    //convert buf to IStream    
    CComPtr<IStream> stream;
    stream.Attach(SHCreateMemStream(buf, bufsize));

    //Read from IStream    
    Gdiplus::Bitmap *image = Gdiplus::Bitmap::FromStream(stream);
    if (image)
    {
        Gdiplus::Graphics g(hdc);
        g.DrawImage(image, 0, 0);
        delete image;
    }

    delete[]buf;
    CloseHandle(hfile);
}

Edit: easier method as mentioned in comments:

IStream* stream = SHCreateMemStream(buf, bufsize);
Gdiplus::Image *image = Gdiplus::Image::FromStream(stream);
...
stream->Release();