Graphics Reference
In-Depth Information
We can disable and enable rasterization while capturing in transform
feedback mode using
glEnable
and
glDisable
with
GL_RASTERIZER_DISCARD
. While
GL_RASTERIZER_DISCARD
is enabled,
no fragment shader will run.
Note that we describe a full example of using transform feedback in
the
Particle System Using Transform Feedback
example in Chapter 14,
“Advanced Programming with OpenGL ES 3.0.”
OpenGL ES 3.0 supports texture lookup operations in a vertex shader. This
is useful to implement techniques such as displacement mapping, where
you can displace the vertex position along the vertex normal based on
the texture lookup value in the vertex shader. A typical application of the
displacement mapping technique is for rendering terrain or water surfaces.
Performing texture lookup in a vertex shader has some notable
limitations:
• The level of detail is not implicitly computed.
• The bias parameter in the
texture
lookup function is not accepted.
• The base texture is used for mipmapped texture.
The maximum number of texture image units supported by an
implementation can be queried using
glGetIntegerv
with
GL_MAX_VERTEX_TEXTURE_UNITS
. The minimum number that an
OpenGL ES 3.0 implementation can support is 16.
Example 8-8 is a sample vertex shader that performs displacement
mapping. The process of loading textures on various texture units is
described in more detail in Chapter 9, “Texturing.”
Example 8-8
Displacement Mapping Vertex Shader
#version 300 es
// uniforms used by the vertex shader
uniform mat4 u_mvpMatrix; // matrix to convert P from
// model space to clip space
uniform sampler2D displacementMap;
// attribute inputs to the vertex shader
layout(location = 0) in vec4 a_position; // input position value
