Graphics Reference
In-Depth Information
Figure 10-6
User Clip Plane Example
The first thing the shader needs to do is compute the distance to the
plane, as mentioned earlier. This could be done in either the vertex shader
(and passed into a varying) or the fragment shader. It is cheaper in terms
of performance to do this computation in the vertex shader rather than
having to compute the distance in every fragment. The vertex shader
listing in Example 10-5 shows the distance-to-plane computation.
Example 10-5
User Clip Plane Vertex Shader
#version 300 es
uniform vec4 u_clipPlane;
uniform mat4 u_matViewProjection;
in vec4 a_vertex;
out float v_clipDist;
void main( void )
{
// Compute the distance between the vertex and
// the clip plane
v_clipDist = dot( a_vertex.xyz, u_clipPlane.xyz ) +
u_clipPlane.w;
gl_Position = u_matViewProjection * a_vertex;
}
The
u_clipPlane
uniform variable holds the plane equation for the clip
plane and is passed into the shader using
glUniform4f
. The
v_clipDist
varying variable then stores the computed clip distance. This value is passed
into the fragment shader, which uses the interpolated distance to determine
whether the fragment should be clipped, as shown in Example 10-6.
