Texture Alpha

Alpha can also be provided with textures. First, the texture must be created. Then it must be set on a texture stage and the appropriate stage operation and operands selected. Then, the primitive can be drawn with transparency.

// Create an alpha texture
CreateEmptyTexture( "gradient", 128,128,0,D3DTEXTR_32BITSPERPIXEL|D3DTEXTR_CREATEWITHALPHA);

LoadGradient( "gradient" );

The body of these routines does the work. CreateEmptyTexture sets everything up by creating an empty texture.

HRESULT LoadGradient( TCHAR* strName )
{
    TextureContainer* ptcTexture = FindTexture( strName );
 
    // Load a gradient
    return ptcTexture->LoadGradient32( strName );
}

It calls LoadGradient32 to load the alpha channel when it has performed a texture lookup.

HRESULT TextureContainer::LoadGradient32( TCHAR* strPathname )
{
 int  yGrad, xGrad;
 // w x h set in createempty
    m_pRGBAData = new DWORD[m_dwWidth*m_dwHeight];
 
    if( m_pRGBAData == NULL )
    {
        return E_FAIL;
    }
 
    for( DWORD y=0; y < m_dwHeight; y++ )
    {
        DWORD dwOffset = y*m_dwWidth;
        yGrad = (int)(((float)y/(float)m_dwWidth) * 255.0f);
 
        for( DWORD x=0; x < m_dwWidth; x )
        {
            xGrad = (int)(((float)x/(float)m_dwWidth) * 255.0f);
 
            DWORD b = (DWORD)(xGrad + (255 - yGrad))/2 & 0xFF;
            DWORD g = (DWORD)((255 - xGrad) + yGrad)/2 & 0xFF;
            DWORD r = (DWORD)(xGrad + yGrad)/2 & 0xFF;
            DWORD a = (DWORD)(xGrad + yGrad)/2 & 0xFF;
 
            m_pRGBAData[dwOffset+x] = (r<<24L)+(g<<16L)+(b<<8L)+(a);
            x++;
        }
    }
    
    return S_OK;
}

The alpha value is calculated based on the current pixel's relative x/y position within the texture size.

Next, assign the texture to a texture stage and set up the texture stage.

// Assign texture
m_pd3dDevice->SetTexture( 0, D3DTextr_GetSurface("gradient") );

// Texture stage states
m_pd3dDevice->SetTextureStageState( 0, D3DTSS_COLORARG1, D3DTA_TEXTURE );
m_pd3dDevice->SetTextureStageState( 0, D3DTSS_COLORARG2, D3DTA_DIFFUSE );  
 
m_pd3dDevice->SetTextureStageState( 0, D3DTSS_COLOROP, D3DTOP_MODULATE );
 
m_pd3dDevice->SetTextureStageState( 0, D3DTSS_ALPHAARG1, D3DTA_TEXTURE );
m_pd3dDevice->SetTextureStageState( 0, D3DTSS_ALPHAARG2, D3DTA_DIFFUSE );

m_pd3dDevice->SetTextureStageState( 0, D3DTSS_ALPHAOP, D3DTOP_MODULATE );
 
// Draw the lit sphere
m_pd3dDevice->DrawIndexedPrimitive( D3DPT_TRIANGLELIST, D3DFVF_LVERTEX,
                              m_pObstacleVertices, m_dwNumObstacleVertices,
                              m_pObstacleIndices,  m_dwNumObstacleIndices, 0 );

This results in a transparent, textured sphere.