Question

This is my first post here. I have a problem.
I need to take a sceenshot of the desktop, convert it to jpeg, store it in a buffer and then manipulate it and send it over the internet.

I have written the code for doing this with GetDC….and GDI+ for converting the HBITMAP to jpeg. The problem I am having now is that I don’t know the size of the jpeg that has been saved into the IStream. Here is part of the code that transforms the bitmap referenced by the HBITMAP hBackBitmap into jpeg and save it into pStream. I need to know how many bytes have been written into pStream and how I can use pStream (get a PVOID handle):

Gdiplus::Bitmap bitmap(hBackBitmap, NULL);///loading the HBITMAP
CLSID clsid;
GetEncoderClsid(L"image/jpeg", &clsid);
HGLOBAL hGlobal = GlobalAlloc(GMEM_FIXED, nBlockSize) ;//allocating memory, the size of the current bitmap size. i'm over allocating but i don't think there is any way to get the exact ammount I need to allocate, is there?
if(!hGlobal)
    return;
IStream* pStream = NULL ;
if(CreateStreamOnHGlobal(hGlobal, TRUE, &pStream) != S_OK )
    return;
bitmap.Save(pStream, &clsid);

What I need is:
1. Find out the size of jpeg, how many bytes have been written in the stream
2. How to use the stream. Can I get a PVOID for the data in stream for example?

Thank you.

Answer 1

According to the CreateStreamOnHGlobal documentation, your use of it is incorrent. Quote:

The current contents of the memory block are undisturbed by the creation of the new stream object. Thus, you can use this function to open an existing stream in memory.
The initial size of the stream is the size of the memory handle returned by the GlobalSize function.

Therefore, you should replace nBlockSize with 0 to allocate an zero-sized buffer. As the memory buffer must be moveable, you’ll also need to replace GMEM_FIXED with GMEM_MOVEABLE:

HGLOBAL gGlobal = GlobalAlloc(GMEM_MOVEABLE, 0);

After saving to the stream, the resulting size will be available as

size_t size = GlobalSize(hGlobal);

To access the JPEG encoded data, you’ll need to use GlobalLock to get a pointer to the actual location in memory.

Note that Global and Local functions are marked as deprecated and should not be used anymore, but I don’t know a better IStream implementation for your needs without crawling the MSDN documentation. Perhaps somebody else can help here!?