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

Question

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?

Answer 1

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;
}