UWP开发细节记录：加载图像文件到D2D位图和D3D纹理

在UWP中加载文件一般先创建 StorageFile 对象，然后调用StorageFile.OpenReadAsync 方法得到一个IRandomAccessStream 接口用来读取数据：

StorageFile image_file = await StorageFile.GetFileFromApplicationUriAsync(new Uri("ms-appx:///Assets/sample.jpg", UriKind.Absolute));
IRandomAccessStream stream = await image_file.OpenReadAsync();

从一个 IRandomAccessStream 流开始，加载图像的步骤（C++ /CX实现）：

1. 转换 IRandomAccessStream^ 到 ISream 接口：

ComPtr<IStream> source_stream;
DX::ThrowIfFailed(::CreateStreamOverRandomAccessStream(stream, IID_PPV_ARGS(&source_stream)));

2. 调用 IWICImagingFactory::CreateDecoderFromStream 方法（假设IWICImagingFactory对象以创建）创建 IWICBitmapDecoder 对象用于图像解码，并最终得到 IWICFormatConverter 对象转换为适合D2D/D3D的位图格式：

ComPtr<IWICBitmapDecoder> decoder;
DX::ThrowIfFailed(_device_resources->GetWicImagingFactory()->CreateDecoderFromStream(source_stream.Get(), nullptr, WICDecodeMetadataCacheOnDemand, &decoder));

ComPtr<IWICBitmapFrameDecode> frame;
DX::ThrowIfFailed(decoder->GetFrame(0, &frame));

// Convert the image to a pixel format supported by Direct2D. 32bppPBGRA is guaranteed to be supported on all hardware.
ComPtr<IWICFormatConverter> covert;
DX::ThrowIfFailed(_device_resources->GetWicImagingFactory()->CreateFormatConverter(&covert));
DX::ThrowIfFailed(covert->Initialize(frame.Get(), GUID_WICPixelFormat32bppPBGRA, WICBitmapDitherTypeNone, nullptr, 0.0f, WICBitmapPaletteTypeCustom));

3. 如果图像用于 D2D ，现在就可以直接用 ID2D1DeviceContext::CreateBitmapFromWicBitmap 方法创建 D2D 位图了：

ComPtr<ID2D1Bitmap1> d2d_bitmap;
DX::ThrowIfFailed(_device_resources->GetD2DDeviceContext()->CreateBitmapFromWicBitmap(covert.Get(), &d2d_bitmap));

4. 如果要用于 D3D 则需要先创建 D3D 纹理：

// 创建D3D纹理用于3D渲染
D3D11_TEXTURE2D_DESC tex_desc = { 0 };
tex_desc.ArraySize = 1;
tex_desc.BindFlags = D3D11_BIND_RENDER_TARGET | D3D11_BIND_SHADER_RESOURCE;
tex_desc.CPUAccessFlags = 0;
tex_desc.Format = DXGI_FORMAT_B8G8R8A8_UNORM;
tex_desc.Height = height;
tex_desc.Width = width;
tex_desc.MipLevels = 1;
tex_desc.MiscFlags = 0;
tex_desc.SampleDesc.Count = 1;
tex_desc.SampleDesc.Quality = 0;
tex_desc.Usage = D3D11_USAGE_DEFAULT;

ComPtr<ID3D11Texture2D> image_texture;
DX::ThrowIfFailed(_device_resources->GetD3DDevice()->CreateTexture2D(&tex_desc, NULL, &image_texture));

5. 从 ID3D11Texture2D 对象中获取 IDXGISurface 接口，D2D和D3D只能通过DXGI进行交互：

ComPtr<IDXGISurface> dxgi_surface;
DX::ThrowIfFailed(image_texture.As(&dxgi_surface));

6. 通过 ID2D1Factory::CreateDxgiSurfaceRenderTarget 方法创建 D2D 渲染目标：

float dpiX = 1.0f;
float dpiY = 1.0f;
_device_resources->GetD2DFactory()->GetDesktopDpi(&dpiX, &dpiY);

D2D1_RENDER_TARGET_PROPERTIES props = D2D1::RenderTargetProperties(
            D2D1_RENDER_TARGET_TYPE_DEFAULT,
            D2D1::PixelFormat(DXGI_FORMAT_B8G8R8A8_UNORM, D2D1_ALPHA_MODE_PREMULTIPLIED),
            dpiX,
            dpiY
            );

ComPtr<ID2D1RenderTarget> render_target;
DX::ThrowIfFailed(_device_resources->GetD2DFactory()->CreateDxgiSurfaceRenderTarget(dxgi_surface.Get(), props, &render_target));

7. 通过 ID2D1RenderTarget::CreateBitmapFromWicBitmap 方法创建位图（注意这里不能直接使用第3步创建的位图，是不同的渲染目标，设备相关的位图不能通用）：

ComPtr<ID2D1Bitmap> bitmap;
DX::ThrowIfFailed(render_target->CreateBitmapFromWicBitmap(covert.Get(), &bitmap));

8. 绘制位图到渲染目标，实际上也是绘制到 D3D 纹理：

render_target->BeginDraw();
render_target->DrawBitmap(bitmap.Get());
render_target->EndDraw();

参考：MSDN 《Direct2D 和 Direct3D 互操作性概述》