D3D9 Alpha Blending on the surfaces

Question

I have a surface (OffScreenPlain or RenderTarget with D3DFMT_A8R8G8B8) which I copy pixels (ARGB) to, from a third party function. Before pixel copying, Bits are accessed by LockRect.

This surface is then StretchRect to the Backbuffer which is (D3DFMT_A8R8G8B8). Surface and Backbuffer are different dimensions. Filtering is set to D3DTEXF_NONE.

Just after creating the d3d device I've set following RenderState settings

D3DRS_ALPHABLENDENABLE -> TRUE
D3DRS_BLENDOP          -> D3DBLENDOP_ADD
D3DRS_SRCBLEND         -> D3DBLEND_SRCALPHA
D3DRS_DESTBLEND        -> D3DBLEND_INVSRCALPHA

But I see no alpha blending happening. I've verified that alpha is specified in pixels.

I've done a simple test by creating a vertex buffer and drawing a triangle (DrawPrimitive) which displays with alpha blending.

In this test surface was StretchRect first and then DrawPrimitive, and the surface content displays without alpha blending and the triangle displays with alpha blending.

What am I missing here? Thanks

Answer 1

States that you mentioned affect vertexes only. In order to have textures blended correctly you need to specify how texture alpha is blended with vertexes one. You do this like this:

d3ddevice->SetTextureStageState(0, D3DTSS_ALPHAOP, D3DTOP_MODULATE);
d3ddevice->SetTextureStageState(0, D3DTSS_ALPHAARG1, D3DTA_TEXTURE);
d3ddevice->SetTextureStageState(0, D3DTSS_ALPHAARG2, D3DTA_DIFFUSE);

By default ALPHAOP is SELECTARG1, i.e. only texture alpha is considered in rendering.

Another solution is using ID3DXSprite::Draw method for copying texture to the back buffer. Just pass color D3DCOLOR_ARGB(X, 255, 255, 255) where X is your alpha. This method will do all the states adjustments and final result will match your expectations.