Can I use Direct3D to generate thumbnails?

Question

I can use D3DXCreateTextureFromFile to load an image, and D3DXSaveTextureToFile to save it, but how can I resize it? Should I use IDirect3DDevice9::SetRenderTarget and render to a texture?

Is it slower than doing it on CPU?

Answer 1

Generally you 'resize' images with the GPU by drawing them as a 'fullscreen quad' onto a render target set to the target size. Depending on the size and the data involved, it's probably slower to ship it over the GPU and then rely on readback to get it to disk so doing it on the CPU is usually the better approach. I believe the legacy deprecated D3DX9 utility library you are using can do the resize with D3DXLoadSurfaceFromSurface.

You should not be using Direct3D 9 and/or the legacy D3DX9 library for new projects. See MSDN and Living without D3DX

A better solution is to use the Windows Imaging Component directly to load the image, resize it, and save a thumbnail. See DirectXTex for extensive example code using WIC.