Alpha-Blending States

The alpha value of a color controls its transparency. Enabling alpha blending allows colors, materials, and textures on a surface to be blended with transparency onto another surface. For more information, see Alpha Texture Blending and Multipass Texture Blending.

[C++]

Applications written in C++ use the D3DRENDERSTATE_ALPHABLENDENABLE render state to enable alpha transparency blending. The Direct3D API allows many types of alpha blending. However, it is important to note the user's 3-D hardware may not support all of the blending states allowed by Direct3D.

The type of alpha blending that is done depends on the D3DRENDERSTATE_SRCBLEND and D3DRENDERSTATE_DESTBLEND render states. Source and destination blend states are used in pairs. The following code fragment demonstrates how the source blend state is set to D3DBLEND_SRCCOLOR and the destination blend state is set to D3DBLEND_INVSRCCOLOR.

// This code fragment assumes that lpD3DDevice is a valid pointer to
// an IDirect3DDevice7 interface.
 
// Set the source blend state.
lpD3DDevice->SetRenderState(D3DRENDERSTATE_SRCBLEND, 
                            D3DBLEND_SRCCOLOR);
 
// Set the destination blend state.
lpD3DDevice->SetRenderState(D3DRENDERSTATE_DESTBLEND, 
                            D3DBLEND_INVSRCCOLOR);

As a result of the calls in the preceding code fragment, Direct3D performs a linear blend between the source color (the color of the primitive being rendered at the current location) and the destination color (the color at the current location in the frame buffer). This gives an appearance similar to tinted glass. Some of the color of the destination object seems to be transmitted through the source object. The rest of it appears to be absorbed.

Altering the source and destination blend states can give the appearance of emissive objects in a foggy or dusty atmosphere. For instance, if your application models flames, force fields, plasma beams, or similarly radiant objects in a foggy environment, set the source and destination blend states to D3DBLEND_ONE.

Another application of alpha blending is controlling the lighting in a 3-D scene, also called light mapping. Setting the source blend state to D3DBLEND_ZERO and the destination blend state to D3DBLEND_SRCALPHA darkens a scene according to the source alpha information. The source primitive is used as a light map that scales the contents of the frame buffer to darken it when appropriate. This produces monochrome light mapping.

Color light mapping can be achieved by setting the source alpha blending state to D3DBLEND_ZERO and the destination blend state to D3DBLEND_SRCCOLOR.

Direct3D devices provide alpha value stippling if it is supported by the display hardware. See D3DRENDERSTATE_STIPPLEDALPHA. If your application creates an RGB or ramp software emulation device, Direct3D ignores this enumerated value.

[Visual Basic]

Visual Basic applications use the D3DRENDERSTATE_ALPHABLENDENABLE render state to enable alpha transparency blending. The Direct3D API allows many types of alpha blending. However, it is important to note the user's 3-D hardware may not support all of the blending states allowed by Direct3D.

' This code fragment assumes that D3DDevice contains a 
' reference to a Direct3DDevice7 object.
 
' Set the source blend state.
Call D3DDevice.SetRenderState(D3DRENDERSTATE_SRCBLEND, _ 
                              D3DBLEND_SRCCOLOR);
 
' Set the destination blend state.
Call D3DDevice.SetRenderState(D3DRENDERSTATE_DESTBLEND, _ 
                              D3DBLEND_INVSRCCOLOR);

Color light mapping can be achieved by setting the source alpha blending state to D3DBLEND_ZERO and the destination blend state to D3DBLEND_SRCCOLOR.