ScreenCaptureforHWND: How to get the byte data of the captured frame?

[DobyChao]

I've built a DLL project to capture window screens, based on the ScreenCaptureforHWND example. However, all the frames I captured are zeros. How can I get the byte data?
Here's my capture code (repository here):

bool DXGICapture::Grab(BYTE* buffer)
{
	com_ptr<ID3D11Texture2D> backBuffer;
	check_hresult(m_swapChain->GetBuffer(0, guid_of<ID3D11Texture2D>(), backBuffer.put_void()));

	// create staging texture
	D3D11_TEXTURE2D_DESC desc;
	backBuffer->GetDesc(&desc);
	auto width = desc.Width;
	auto height = desc.Height;

	desc.CPUAccessFlags = D3D11_CPU_ACCESS_READ;
	desc.Usage = D3D11_USAGE_STAGING;
	desc.BindFlags = 0;
	desc.MiscFlags = 0;

	com_ptr<ID3D11Texture2D> stagingTexture { nullptr };
	check_hresult(m_d3dDevice->CreateTexture2D(&desc, nullptr, stagingTexture.put()));

	m_d3dContext->CopyResource(stagingTexture.get(), backBuffer.get());

	D3D11_MAPPED_SUBRESOURCE resource;
	check_hresult(m_d3dContext->Map(stagingTexture.get(), NULL, D3D11_MAP_READ, 0, &resource));

	const size_t bufferSize = width * height * 4;
	memcpy(buffer, resource.pData, bufferSize);

	m_d3dContext->Unmap(stagingTexture.get(), 0);

	return true;
}

[robmikh]

First, make sure you're setting the CPUAccessFlags to D3D11_CPU_ACCESS_READ. Second, I wouldn't bother with the swap chain unless you intend to put the swap chain on screen. The sample uses it purely to show you what the capture looks like. You probably want to use the capture API like this: https://github.com/robmikh/CaptureVideoSample/blob/master/CaptureVideoSample/CaptureFrameGenerator.cpp

When frames come in they will be queued up, and calling TryGetNextFrame will give you a queued frame or block until it receives one.

[robmikh]

Oh, one more thing, you'll want to observe the RowPitch field from the D3D11_MAPPED_SUBRESOURCE struct when accessing the bytes of the texture. The stride of your texture may exceed the width. See this helper method:

https://github.com/robmikh/robmikh.common/blob/010a4149e86db95dd9f6957ca99f4311ab8bcbcb/robmikh.common/include/robmikh.common/d3d11Helpers.h#L206-L237

[DobyChao]

Thanks so much for your reply! I've updated my code here based on your suggestions, but still can't capture the image. The OnFrameArrived function doesn't seem to be triggered. What should I do next?
Here‘s my test code:

// add DXGICapture.lib to library

#include "pch.h"
#include <iostream>
#include "DXGICapture.h"

int main()
{
	HWND hwnd = FindWindow(NULL, L"Ice.mp4 - PotPlayer");
	if (hwnd == NULL)
	{
		std::cout << "FindWindow failed" << std::endl;
		return 1;
	}
	auto cap = CreateCapture(hwnd);
	StartCapture(cap);
	BYTE* buffer = new BYTE[1920 * 1080 * 4];
	Grab(cap, buffer);
	// delay
	Sleep(1000);
}

[robmikh]

Since you're calling Direct3D11CaptureFramePool::Create, you need to be pumping messages. Given that you're using a lock in the FrameArrived callback, you should call Direct3D11CaptureFramePool::CreateFreeThreaded instead. That removes the message pumping requirement and causes the FrameArrived callback to fire on an internal worker thread.