Light Direction in Word

Draw Data Matrix in Word Light Direction

Figure 6-1. Triangles lit according to incoming light Simply defining the position of the light source and the position of your objects is not enough for your graphics card to add correct lighting to an object, though. For each triangle of your 3D object, you will need to add some information, allowing your graphics card to calculate the amount of light hitting the surface. You can do this by specifying the normal vector in each vertex of your object, which are the spikes in the corners of the triangles shown in Figure 6-1. Once you ve specified the correct normal in each vertex, the BasicEffect can render your object with correct lighting.

In Figure 6-1, several squares (each consisting of two triangles) are drawn, and their colors represent the amount of incoming light. The more the square is perpendicular to the direction of the light, the more light it receives. The last square is completely perpendicular to the direction of the light so should be fully lit. The first square, however, is positioned along the direction of the light and should therefore receive no light at all.

How is the graphics card supposed to know this In each corner of each triangle (called a vertex; see recipe 5-1), you ll define the direction perpendicular to the triangle. This direction is called the normal. This normal direction has been indicated in each vertex of Figure 6-1 as a spike. Because there is only one direction perpendicular to a surface, all vertices of the same quad have the same normal direction. This normal direction allows your graphics card to calculate how much light hits the triangle. As explained in detail in recipe 6-5, you do this by projecting the normal on the direction of the light, as shown in Figure 6-2. The direction of the light is shown as the long black arrow at the bottom of the image going from left to right. The rotated black bars in Figure 6-2 represent the quads of Figure 6-1. The projections of the normal of each quad on the light direction are shown as the thick black blocks on the light direction. The bigger the black block, the more your triangle should be lit.

Figure 6-2. Projecting the normal on the light direction The normal of the triangle on the left is perpendicular to the direction of the light, so the projection is 0, and the triangle will not be lit. The triangle on the right side has a normal parallel to the light direction, so its projection will be maximal, and the plane will be lit at full intensity. Given the direction of the light and the normal in a vertex, your graphics card can easily calculate the length of the projection (the thick black block). This is how the graphics card calculates the correct lighting.

For each pixel, the graphics card will calculate the amount of lighting that should be applied to that pixel, as described earlier. Next, the graphics card multiplies this amount of lighting with the original color of that pixel.

The previous paragraph explains that, next to the 3D position and color, each vertex should also store its normal direction. XNA comes with one predefined vertex format that allows you to save a normal in each vertex: the VertexPositionNormalTexture struct. This format allows you to save the 3D position, normal direction, and texture coordinate for each vertex. See recipe 5-2 on textured triangles and recipe 5-14 to learn how you can define your own vertex format. The following method creates an array to hold six such vertices, which define two triangles that create a quad. This quad is lying on the floor, because all Y coordinates of the vertices are 0. Therefore, the normal stored in each vertex is the (0,1,0) Up direction, because this is the direction perpendicular to the quad. The vertex format also expects you to add texture coordinates, so the graphics card knows where to sample the colors from an image (see recipe 5-2). private void InitVertices() { vertices = new VertexPositionNormalTexture[6]; int i = 0; vertices[i++] = new VertexPositionNormalTexture(new Vector3(-1, 0, 1), new Vector3(0, 1, 0), new Vector2(1,1)); vertices[i++] = new VertexPositionNormalTexture(new Vector3(-1, 0, -1), new Vector3(0, 1, 0), new Vector2(0,1)); vertices[i++] = new VertexPositionNormalTexture(new Vector3(1, 0, -1), new Vector3(0, 1, 0), new Vector2(0,0)); vertices[i++] = new VertexPositionNormalTexture(new Vector3(1, 0, -1), new Vector3(0, 1, 0), new Vector2(0,0)); vertices[i++] = new VertexPositionNormalTexture(new Vector3(1, 0, 1), new Vector3(0, 1, 0), new Vector2(1,0)); vertices[i++] = new VertexPositionNormalTexture(new Vector3(-1, 0, 1), new Vector3(0, 1, 0), new Vector2(1,1)); myVertexDeclaration = new VertexDeclaration(device, VertexPositionNormalTexture.VertexElements); } The last line makes sure the VertexDeclaration (see recipe 5-1) is created only once, because this will not need to be changed.

Tip In this case of a triangle lying flat on the ground, it s easy to calculate its normal. Read recipe 5-7 to