Graphics Reference
In-Depth Information
// Bind the light map
glActiveTexture(GL_TEXTUREl);
glBindTexture(GL_TEXTURE_2D, userData->lightMapTexId);
// Set the light map sampler to texture unit 1
glUniformli(userData->lightMapLoc, 1);
As you can see, this code binds each of the individual texture objects
to textures units 0 and 1. The samplers are set with values to bind the
samplers to the respective texture units. In this example, a single texture
coordinate is used to fetch from both of the maps. In typical light
mapping, there would be a separate set of texture coordinates for the base
map and light map. The light maps are typically paged into a single large
texture and the texture coordinates can be generated using offline tools.
Fog
A common technique that is used in rendering 3D scenes is the
application of fog. In OpenGL ES 1.1, fog was provided as a fixed-function
operation. One of the reasons fog is such a prevalent technique is that it
can be used to reduce draw distances and remove “popping” of geometry
as it comes in closer to the viewer.
There are a number of possible ways to compute fog, and with programmable
fragment shaders you are not limited to any particular equation. Here we
show how you would go about computing linear fog with a fragment shader.
To compute any type of fog, we will need two inputs: the distance of the
pixel to the eye and the color of the fog. To compute linear fog, we also need
the minimum and maximum distance range that the fog should cover.
The equation for the linear fog factor
MaxDist
−
−
EyeDist
F
=
MaxDist
MinDist
computes a linear fog factor to multiply the fog color by. This color gets
clamped in the [0.0, 1.0] range and then is linear interpolated with the
overall color of a fragment to compute the final color. The distance to
the eye is best computed in the vertex shader and interpolated across the
primitive using a varying variable.
A PVRShaman (.POD) workspace is provided as an example in the
Chapter_10/PVR_LinearFog
folder that demonstrates the fog
computation. Figure 10-4 is a screenshot of the workspace. PVRShaman is
a shader development integrated development environment (IDE) that is
part of the Imagination Technologies PowerVR SDK downloadable from
http://powervrinsider.com/.
Several subsequent examples in the topic use
PVRShaman to demonstrate various shading techniques.
