Creating Effects

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Now that you've seen how to create stencil buffers and configure how they work, let's look at some of the effects you can render with them. The following sections describe several ways Microsoft recommends using stencil buffers. Each of these approaches produces impressive results, but a few of them have drawbacks.

Composites

You can use stencil buffers for compositing 2D or 3D images onto a 3D scene. By using a mask in the stencil buffer to occlude a portion of the render-target surface, you can write stored 2D information (such as text or bitmaps). You can also render 3D primitives—or for that matter a complete scene—to the area of the render-target surface that you specify in a stencil mask.

Developers often use this effect to composite several scenes in simulations and games. Many driving games feature a rear view mirror that displays the scene behind the driver. You can composite this second 3D scene with the driver's view forward by using a stencil to block the portion to which you want the mirror image rendered. You can also use composites to create 2D "cockpits" for vehicle simulations by combining a 2D, bitmapped image of the cockpit with the final, rendered 3D scene.

Decals

You can use decals to control which pixels from a primitive image you draw to a render-target surface. When you apply a texture to an object (for example, applying scratch marks to a floor), you need the texture (the scratch marks) to appear immediately on top of the object (the floor). Because the z values of the scratch marks and the floor are equal, the depth buffer might not yield consistent results, meaning that some pixels in the back primitive might be rendered on top of those in the front primitive. This overlap, which is commonly known as z-fighting or flimmering, can cause the final image to shimmer as you animate from one frame to the next.

You can prevent flimmering by using a stencil to mask the section of the back primitive on which you want the decal to appear. You can then turn off z-buffering and render the image of the front primitive into the masked area of the render-target surface.

Dissolves

You can use dissolves to gradually replace an image by displaying a series of frames that transition from one image to another. In Chapter 8, you saw how to use multiple-texture blending to create this effect by gradually blending two textures together. Stencil buffers allow you to produce similar dissolves, except that a stencil-based dissolve looks more pixelated than a multiple-texture blending one. However, stencil buffers let you use texture-blending capabilities for other effects while performing a dissolve. This capability enables you to efficiently produce more complex effects than you could by using texture blending alone.

A stencil buffer can perform a dissolve by controlling which pixels you draw from two different images to the render-target surface. You can perform a dissolve by defining a base stencil mask for the first frame and altering it incrementally or by defining a series of stencil masks and copying them into the stencil buffer on successive frames.

To start a dissolve, set the stencil function and stencil mask so that most of the pixels from the starting image pass the stencil test and most of the ending image's pixels fail. For each subsequent frame, update the stencil mask to allow fewer pixels in the starting image to pass the test and more pixels in the ending image to pass. By controlling the stencil mask, you can create a variety of dissolve effects.

Although this approach can produce some fantastic effects, it can be a bit slow on some systems. You should test the performance on your target systems to verify that this approach works efficiently for your application.

Fades

You can fade in or out using a form of dissolving. To perform this effect, use any dissolve pattern you want. To fade in, use a stencil buffer to dissolve from a black or white image to a rendered 3D scene. To fade out, start with a rendered 3D scene and dissolve to black or white. As with dissolves, you should check the performance of fades on the target systems to verify that their speed and appearance is acceptable.

Outlines

You can apply a stencil mask to a primitive that's the same shape but slightly smaller than the primitive. The resulting image will contain only the primitive's outline. You can then fill this stencil-masked area of the primitive with a color or set of colors to produce an outline around the image.

Silhouettes

When you set the stencil mask to the same size and shape as the primitive you're rendering, Direct3D produces a final image containing a "black hole" where the primitive should be. By coloring this hole, you can produce a silhouette of the primitive.

Swipes

A swipe makes an image appear as though it's sliding into the scene over another image. You can use stencil masks to disable the writing of pixels from the starting image and enable the writing of pixels from the ending image. To perform a swipe, you can define a series of stencil masks that Direct3D will load into the stencil buffer in a succession of frames, or you can change the starting stencil mask for a series of successive frames. Both methods cause the final image to look as though it's gradually sliding on top of the starting image from right to left, left to right, top to bottom, and so on.

To handle a swipe, remember to read the pixels from the ending image in the reverse order in which you're performing the swipe. For example, if you're performing a swipe from left to right, you need to read pixels from the ending image from right to left. As with dissolves, this effect can render somewhat slowly. Therefore, you should test its performance on your target systems.

Shadows

Shadow volumes, which allow an arbitrarily shaped object to cast a shadow onto another arbitrarily shaped object, can produce some incredibly realistic effects. To create shadows with stencil buffers, take an object you want to cast a shadow. Using this object and the light source, build a set of polygonal faces (a shadow volume) to represent the shadow.

You can compute the shadow volume by projecting the vertices of the shadow-casting object onto a plane that's perpendicular to the direction of light from the light source, finding the 2D convex hull of the projected vertices (that is, a polygon that "wraps around" all the projected vertices), and extruding the 2D convex hull in the light direction to form the 3D shadow volume. The shadow volume must extend far enough so that it covers any objects that will be shadowed. To simplify computation, you might want the shadow caster to be a convex object.

To render a shadow, you must first render the geometry and then render the shadow volume without writing to the depth buffer or the color buffer. Use alpha blending to avoid having to write to the color buffer. Each place that the shadow volume appears will be marked in the stencil buffer. You can then reverse the cull and render the backfaces of the shadow volume, unmarking all the pixels that are covered in the stencil buffer. All these pixels will have passed the z-test, so they'll be visible behind the shadow volume. Therefore, they won't be in shadow. The pixels that are still marked are the ones lying inside the front and back boundaries of the shadow volume—these pixels will be in shadow. You can blend these pixels with a large black rectangle that covers the viewport to generate the shadow.