How to mix textures in DirectX?

Question

I am new to DirectX development and I am wondering if I am taking the wrong route to achieve the following:

I would like to mix three textures which contain transparent areas and some solid areas (Red, Blue, Green). The three textures should blend like shown in this example:

How can I achieve that in DirectX (preferably in directx9)?

A link or example code would be nice.

Update:

My rendering method looks like this and I still think I am doing it wrong, because the sprite only shows the last texture (nothing is rendered transparent or blended):

void D3DTester::render()
{
d3ddevice->Clear(0, NULL, D3DCLEAR_TARGET, D3DCOLOR_XRGB(0,0,0), 1.0f, 0);
d3ddevice->BeginScene();

d3ddevice->SetRenderState(D3DRS_ALPHABLENDENABLE, TRUE);
d3ddevice->SetRenderState(D3DRS_SRCBLEND, D3DBLEND_ONE);
d3ddevice->SetRenderState(D3DRS_DESTBLEND, D3DBLEND_ONE);

LPD3DXSPRITE sprite=NULL;
HRESULT hres = D3DXCreateSprite(d3ddevice, &sprite);
if(hres != S_OK)
{
    throw std::exception(); 
}

sprite->Begin(D3DXSPRITE_ALPHABLEND);

std::vector<LPDIRECT3DTEXTURE9>::iterator it;
for ( it=textures.begin() ; it < textures.end(); it++ )
{
    sprite->Draw(*it, NULL, NULL, NULL, 0xFFFFFFFF);
}

sprite->End();

d3ddevice->EndScene();
d3ddevice->Present(NULL, NULL, NULL, NULL);
}

The resulting image looks like this:

But I need it to look like this instead:

Update2: I figured out that I have to SetRenderState after I use sprite->Begin(D3DXSPRITE_ALPHABLEND); thanks to the hint by Josh Petrie. However, by using this:

    sprite->Begin(D3DXSPRITE_ALPHABLEND);

d3ddevice->SetRenderState(D3DRS_ALPHABLENDENABLE, TRUE);
d3ddevice->SetRenderState(D3DRS_SRCBLEND, D3DBLEND_ONE);
d3ddevice->SetRenderState(D3DRS_DESTBLEND, D3DBLEND_ONE);

std::vector<LPDIRECT3DTEXTURE9>::iterator it;
for ( it=textures.begin() ; it < textures.end(); it++ )
{   
    sprite->Draw(*it, NULL, NULL, NULL, 0xFFFFFFFF);
}
sprite->End();      

The sprites colors are becoming transparent towards the background scene e.g.: if I use d3ddevice->Clear(0, NULL, D3DCLEAR_TARGET, D3DCOLOR_XRGB(0,100,21), 1.0f, 0); the result looks like:

Is there any way to avoid that? I would like the sprites be transparent to each other but to be still solid to the background.

Answer 1

This is known as Additive Blending.

In DirectX9, you use a few render states to set that up:

pDirect3DDevice9->SetRenderState(D3DRENDERSTATE_ALPHABLENDENABLE, TRUE);
pDirect3DDevice9->SetRenderState(D3DRENDERSTATE_SRCBLEND, D3DBLEND_ONE);
pDirect3DDevice9->SetRenderState(D3DRENDERSTATE_DESTBLEND, D3DBLEND_ONE);

Answer 2

The ID3DXSprite interface will modify the render states of the device when you call the Begin method. The specifics of the state modifications that will be performed are detailed in the method's documentation. In particular, note that the source and destination blend modes are set to 'source alpha' and 'inverse source alpha,' respectively. This enables the typical blending behavior wherein the alpha component of the color is used to control how transparent a pixel is.

Since this isn't what you want, you should pass the D3DXSPRITE_DONOTMODIFY_RENDERSTATE flag to Begin and ensure, as the documentation notes, that

The device is assumed to be in a valid state to draw vertices containing UsageIndex = 0 in the D3DDECLUSAGE_POSITION, D3DDECLUSAGE_TEXCOORD, and D3DDECLUSAGE_COLOR data.

By doing this, the render states you set up to blend the way you want will be preserved. You may note that doing this changes some other aspect of your render behavior, and that is probably due to your not setting other render states to the values described in the documentation for Begin.

As an aside, you shouldn't test HRESULT values against specific constants (like S_OK) if all you want to know is success or failure -- as is the case in your code. There are multiple success or failure codes. Prefer the SUCCEEDED or FAILED macro as appropriate.