How to create a transparent bitmap in Direct2D

问题

I need to create a transparent bitmap using Direct2D and draw, by using my device context, on it.

ID2D1DeviceContext1* d2dContext = ...
ID2D1Bitmap* pBitmap;

d2dContext->CreateBitmap(
    bitmapSize,
    nullptr,
    0,
    D2D1::BitmapProperties1(
        D2D1_BITMAP_OPTIONS_TARGET,
        D2D1::PixelFormat(DXGI_FORMAT_B8G8R8A8_UNORM, D2D1_ALPHA_MODE_PREMULTIPLIED),
        dpiX, dpiY),
    &pBitmap);

d2dContext->BeginDraw();
d2dContext->SetTarget(pBitmap);
d2dContext->Clear(D2D1::ColorF(0, 0));
d2dContext->DrawLine(...);
hr = d2dContext->EndDraw();

Unfortunately I am not able to create any transparent bitmaps. I tried with several pixel format combinations, including D2D1_ALPHA_MODE_STRAIGHT, but without succeeding.

Is there any solution?

回答1:

Good news Nick: You are creating transparent bitmaps, however the reason why you are not seeing your expected results has to do with window creation. Kenny Kerr demonstrated proper layered window creation with Direct2D back in 2009 here.

Since then much has changed with respect to windows rendering, with Win8 and Win10 using a composition engine, and allowing developers access to the DirectComposition API. As a result, Kenny Kerr provided an updated article in 2014 on the topic.

My recent projects required Win7 support, so I personally stick with pure Direct2D.

EDIT: And of course ensure your render target is properly created. Hope this helps.

回答2:

Here is what I do in Sciter when I need to create cached rendering (bitmap) :

ID2D1RenderTarget* src = ...
d2d::asset<ID2D1BitmapRenderTarget> dst = nullptr;

// creating temp surface - compatible render target:
src->CreateCompatibleRenderTarget(sz,dst.target());

dst->BeginDraw();
dst->Clear(init_color);

... drawing on that temp surface ...
dst->EndDraw();

d2d::asset<ID2D1Bitmap> dst_bmp;
hr = dst->GetBitmap(il->d2d_bmp.target());

return dst_bmp;

This method delegates bitmap creation to the ID2D1RenderTarget and so that bitmap will always be compatible with source. The code is used with transparent init_color's mostly.

I wouldn't use SetTarget() as it is not clear what it does with clips, etc.

来源：https://stackoverflow.com/questions/39900556/how-to-create-a-transparent-bitmap-in-direct2d

标签

c++

directx

transparency

direct2d

pixelformat