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C++ gdi::Bitmap to PNG Image in memory

I'm trying to send a screenshot of a window over tcp to a server. Getting the screenshot is no problem (using GDIplus). The networking is also easy for me. The problem is trying to convert the gdi+ Bitmap to a png (in memory) to get the data out of it and send it to the server. Can anyone help me please?

like image 903
surreal Avatar asked Jul 17 '18 14:07

surreal


1 Answers

Gdiplus can save to file, or save to memory using IStream. See Gdiplus::Image::Save method

//get gdi+ bitmap
Gdiplus::Bitmap bitmap(hbitmap, nullptr);

//write to IStream
IStream* istream = nullptr;
HRESULT hr = CreateStreamOnHGlobal(NULL, TRUE, &istream);
CLSID clsid_png;
CLSIDFromString(L"{557cf406-1a04-11d3-9a73-0000f81ef32e}", &clsid_png);
bitmap.Save(istream, &clsid_png);

The memory size is small enough that you can copy from IStream to a single buffer (see "Minimum example" for more detail)

//get memory handle associated with istream
HGLOBAL hg = NULL;
GetHGlobalFromStream(istream, &hg);

//copy IStream to buffer
int bufsize = GlobalSize(hg);
char *buffer = new char[bufsize];

//lock & unlock memory
LPVOID ptr = GlobalLock(hg);
memcpy(buffer, ptr, bufsize);
GlobalUnlock(hg);

//release will automatically free the memory allocated in CreateStreamOnHGlobal 
istream->Release();

PNG is now available in buffer, its size is bufsize. You can work directly with the binary data, or convert to Base64 to send over the network.

Minimum example:

#include <iostream>
#include <fstream>
#include <vector>
#include <Windows.h>
#include <gdiplus.h>

bool save_png_memory(HBITMAP hbitmap, std::vector<BYTE>& data)
{
    Gdiplus::Bitmap bmp(hbitmap, nullptr);

    //write to IStream
    IStream* istream = nullptr;
    if (CreateStreamOnHGlobal(NULL, TRUE, &istream) != 0)
        return false;

    CLSID clsid_png;
    if (CLSIDFromString(L"{557cf406-1a04-11d3-9a73-0000f81ef32e}", &clsid_png)!=0)
        return false;
    Gdiplus::Status status = bmp.Save(istream, &clsid_png);
    if (status != Gdiplus::Status::Ok)
        return false;

    //get memory handle associated with istream
    HGLOBAL hg = NULL;
    if (GetHGlobalFromStream(istream, &hg) != S_OK)
        return 0;

    //copy IStream to buffer
    int bufsize = GlobalSize(hg);
    data.resize(bufsize);

    //lock & unlock memory
    LPVOID pimage = GlobalLock(hg);
    if (!pimage)
        return false;
    memcpy(&data[0], pimage, bufsize);
    GlobalUnlock(hg);

    istream->Release();
    return true;
}

int main()
{
    CoInitialize(NULL);

    ULONG_PTR token;
    Gdiplus::GdiplusStartupInput tmp;
    Gdiplus::GdiplusStartup(&token, &tmp, NULL);

    //take screenshot
    RECT rc;
    GetClientRect(GetDesktopWindow(), &rc);
    auto hdc = GetDC(0);
    auto memdc = CreateCompatibleDC(hdc);
    auto hbitmap = CreateCompatibleBitmap(hdc, rc.right, rc.bottom);
    auto oldbmp = SelectObject(memdc, hbitmap);
    BitBlt(memdc, 0, 0, rc.right, rc.bottom, hdc, 0, 0, SRCCOPY);
    SelectObject(memdc, oldbmp);
    DeleteDC(memdc);
    ReleaseDC(0, hdc);

    //save as png
    std::vector<BYTE> data;
    if(save_png_memory(hbitmap, data))
    {
        //write from memory to file for testing:
        std::ofstream fout("test.png", std::ios::binary);
        fout.write((char*)data.data(), data.size());
    }
    DeleteObject(hbitmap);

    Gdiplus::GdiplusShutdown(token);
    CoUninitialize();

    return 0;
}
like image 50
Barmak Shemirani Avatar answered Sep 27 '22 21:09

Barmak Shemirani