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

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();