Introduction to 3D Game Programming with DirectX.9.0 - F. D. Luna

144 Chapter 8

Finally, we are ready to draw the reflected teapot:

Device->SetTransform(D3DTS_WORLD, &W);

Device->SetMaterial(&TeapotMtrl);

Device->SetTexture(0, 0);

Device->SetRenderState(D3DRS_CULLMODE, D3DCULL_CW);

Teapot->DrawSubset(0);

Recall from section 8.2.3.4 that W correctly transforms the reflected

teapot into its appropriate position in the scene. Also, observe that we change the backface cull mode. We must do this because when an

faces will have a winding order that indicatesYto Direct3D that they are

object is reflected, its front faces will be swapped with its back faces;

however, the winding order will not be changed. Thus, the “new” front

Device->SetRenderState(D3DRS_ALPHABLENDENABLE, false);

Device->SetRenderState( D3DRS_STENCILENABLE, false);

Device->SetRenderState(D3DRS_CULLMODE, D3DCULL_CCW);

}// end RenderMirror()

8.3Sample Application: Planar Shadows

Shadows aid in our perception of where light is being emitted in a scene and ultimately makes the scene more realistic. In this section we show how to implement planar shadows—that is, shadows that lie on a plane (see Figure 8.5).

Figure 8.5: A screen shot taken from this chapter’s sample application.

Notice the teapot’s shadow on the floor.

Team-Fly®

Stenciling 145

146 Chapter 8

Then:

8.3.2 Point Light Shadows

Figure 8.7: The shadow cast with respect to a point light source

Figure 8.7 shows the shadow that an object casts with respect to a point light source whose position is described by the point L. The light ray from a point light through any vertex p is given by r(t) = p + t(p – L). The intersection point of the ray r(t) with the plane n p + d = 0 gives s. The set of intersection points found by shooting r(t) through each of the object’s vertices with the plane defines the geometry of the shadow. s can be solved for using the same technique (plane/ray intersection) used in section 8.3.1.

Note: Notice that L serves different purposes for point and parallel lights. For point lights we use L to define the position of the point light. For parallel lights we use L to define the direction of the parallel light rays.

8.3.3 The Shadow Matrix

Notice from Figure 8.6 that for a parallel light, the shadow is essentially a parallel projection of the object onto the plane n p + d = 0 in the specified light direction. Similarly, Figure 8.7 shows that for a point light, the shadow is essentially a perspective projection of the object onto the plane n p + d = 0 from the viewpoint of the light source.

We can represent the transformation from a vertex p to its projection s with the plane n p + d = 0 with a matrix. Moreover, we can represent both an orthogonal projection and a perspective projection with the same matrix using some ingenuity.

Stenciling 147

148 Chapter 8

Figure 8.8: Notice the “black” areas of the shadow in (a). These correspond to areas where parts of the flattened teapot overlap, causing a “double blend.” Image (b) shows the shadow rendered correctly without double blending.

We can solve this problem using the stencil buffer. We set the stencil test to only accept pixels the first time they are rendered. That is, as we render the shadow’s pixels to the back buffer, we mark the corresponding stencil buffer entries. Then, if we attempt to write a pixel to an area that has already been rendered to (marked in the stencil buffer), the stencil test will fail. In this way, we prevent writing overlapping pixels and therefore avoid double blending artifacts.

8.3.5 Code and Explanation

The following code explanation is taken from the Shadow sample in the companion files. The code relevant to this sample lies in the RenderShadow function. Note that we assume the stencil buffer has already been cleared to zero.

We begin by setting the stencil render states. We set the stencil comparison function to D3DCMP_EQUAL and the D3DRS_STENCILREF render state to 0x0, thereby specifying to render the shadow to the back buffer if the corresponding entry in the stencil buffer equals 0x0.

Since the stencil buffer is cleared to zero (0x0), this will always be true the first time we write a particular pixel of the shadow; but because we set D3DRS_STENCILPASS to D3DSTENCILOP_INCR, the test will fail if we try to write to a pixel that we have already written to. The pixel’s stencil entry will have been incremented to 0x1 the first time it was written to, and thus the stencil test will fail if we try to

150 Chapter 8

D3DMATERIAL9 mtrl = d3d::InitMtrl(d3d::BLACK, d3d::BLACK, d3d::BLACK, d3d::BLACK, 0.0f);

mtrl.Diffuse.a = 0.5f; // 50% transparency.

//Disable depth buffer so that z-fighting doesn't occur when we

//render the shadow on top of the floor.

Device->SetRenderState(D3DRS_ZENABLE, false);

Device->SetMaterial(&mtrl);

Device->SetTexture(0, 0);

Teapot->DrawSubset(0);

Device->SetRenderState(D3DRS_ZENABLE, true);

Device->SetRenderState(D3DRS_ALPHABLENDENABLE, false);

Device->SetRenderState(D3DRS_STENCILENABLE, false);

}//end RenderShadow()

8.4Summary

The stencil buffer and depth buffer share the same surface and are therefore created at the same time. We specify the format of the depth/stencil surface using the D3DFORMAT types.

Stenciling is used to block certain pixels from being rasterized. As we have seen in this chapter, this ability is useful for implementing mirrors and shadows among other applications.

We can control stenciling operations and how the stencil buffer is updated through the D3DRS_STENCIL* render states.

Some other applications that can be implemented with the stencil buffer:

Shadow volumes

Dissolves and fades

Visualizing depth complexity

Outlines and silhouettes

Constructive solid geometry

Fixing z-fighting caused by coplanar geometry

152 Part III

Chapter 15, “Picking”—This chapter shows how to determine the particular 3D object that the user has selected with the mouse. Picking is often a necessity in 3D games and applications where the user interacts with the 3D world with the mouse.