Direct2D CreateBitmap的使用

当需要设置位图的混合模式时，应该使用ID2D1DeviceContext而不是ID2D1RenderTarget。

代码如下：

#define WIN32_LEAN_AND_MEAN
#include <windows.h>
#include <comdef.h>
#include <d2d1.h>
#include <d2d1_1.h>
#include <d3d11.h>

#pragma comment(lib,"D2d1.lib")
#pragma comment(lib,"D3D11.lib")

#include <iostream>
#include <string>

void printError(const std::string& msg, HRESULT err)
{
    std::cerr << "[ERROR] " << msg << ": " << _com_error(err).ErrorMessage() << std::endl;
    assert(false);
}

int main(int argc, char* argv[])
{
    ID2D1Factory1* mD2DFactory = nullptr;
    ID3D11Device* mD3DDevice = nullptr;
    ID3D11DeviceContext* mD3DDeviceContext = nullptr;
    IDXGIDevice* mDXGIDevice = nullptr;
    ID2D1Device* mD2DDevice = nullptr;

    HRESULT err;

    // Create factories first; they don't depend on anything.
    err = D2D1CreateFactory(D2D1_FACTORY_TYPE_SINGLE_THREADED,
        __uuidof(ID2D1Factory1),  // get a Direct2D 1.1 factory,
        (void**)&mD2DFactory);
    if (err != S_OK) {
        printError("fatal: Could not create Direct2D factory!", err);
        return 1;
    }

    // Initialize DirectX 11.1
    D3D_FEATURE_LEVEL featureLevels[] = { D3D_FEATURE_LEVEL_11_1 };
    err = D3D11CreateDevice(NULL,                                 // first adapter
        D3D_DRIVER_TYPE_HARDWARE,
        NULL,                                 // software rasterizer DLL handle
        D3D11_CREATE_DEVICE_SINGLETHREADED    // better performance
        | D3D11_CREATE_DEVICE_BGRA_SUPPORT, // required for Direct2D
        featureLevels,
        sizeof(featureLevels) / sizeof(D3D_FEATURE_LEVEL),
        D3D11_SDK_VERSION,                    // docs say use this
        &mD3DDevice,
        NULL,                                 // don't care what feature level we got
        &mD3DDeviceContext);
    if (err != S_OK) {
        printError("fatal: could not create a Direct3D 11.1 device", err);
        return 1;
    }

    err = mD3DDevice->QueryInterface(__uuidof(IDXGIDevice), (void**)&mDXGIDevice);
    if (err != S_OK) {
        printError("fatal: Direct3D device is not IDXGIDevice", err);
        return 1;
    }

    // Initialize Direct2D
    err = mD2DFactory->CreateDevice(mDXGIDevice, &mD2DDevice);
    if (err != S_OK) {
        printError("fatal: could not create Direct2D device", err);
        return 1;
    }

    //---------------- Create the bitmap --------------------
    ID2D1DeviceContext* mDC = nullptr;
    ID2D1Bitmap1* mBitmap = nullptr;
    int width = 13, height = 13;
    float dpi = 76.0f;

    err = mD2DDevice->CreateDeviceContext(D2D1_DEVICE_CONTEXT_OPTIONS_NONE, &mDC);
    if (err) {
        printError("Could not create device context", err);
        return 1;
    }

    D2D1_BITMAP_PROPERTIES1 bitmapProps;
    bitmapProps.pixelFormat.format = DXGI_FORMAT_B8G8R8A8_UNORM;
    bitmapProps.pixelFormat.alphaMode = D2D1_ALPHA_MODE_PREMULTIPLIED;
    bitmapProps.dpiX = dpi;
    bitmapProps.dpiY = dpi;
    bitmapProps.bitmapOptions = D2D1_BITMAP_OPTIONS_CANNOT_DRAW |  // can use for SetTarget()
        D2D1_BITMAP_OPTIONS_CPU_READ;
    bitmapProps.colorContext = nullptr;

    err = mDC->CreateBitmap({ UINT32(width), UINT32(height) },
        nullptr, 0,
        bitmapProps, &mBitmap);
    if (err == S_OK) {
        mDC->SetTarget(mBitmap);
    }
    else {
        printError("Could not create bitmap (" + std::to_string(width) + "x" + std::to_string(height) + ")", err);
        return 1;
    }

    std::cout << "Success!" << std::endl;
    return 0;
}

类似的例子：per-pixel transparent window using Windows Composition engine in C++