Graphics Reference
In-Depth Information
•
gl_MaxTextureImageUnits
—The maximum number of texture image
units that are available. The minimum value supported by all ES 3.0
implementations is 16.
•
gl_MaxFragmentUniformVectors
—The maximum number of
vec4
uniform entries that can be used inside a fragment shader.
The minimum value supported by all ES 3.0 implementations is
224. The number of
vec4
uniform entries that can actually be used
by a developer can vary from implementation to implementation
and from one fragment shader to another. This issue is described in
Chapter 8, “Vertex Shaders,” and the same issue applies to fragment
shaders.
•
gl_MaxDrawBuffers
—The maximum number of multiple render
targets (MRTs) supported. The minimum value supported by all ES 3.0
implementations is 4.
•
gl_MinProgramTexelOffset/gl_MaxProgramTexelOffset
—The
minimum and maximum offsets supported by the offset parameter to
the
texture*Offset()
built-in ESSL functions.
The values specified for each built-in constant are the minimum values
that must be supported by all OpenGL ES 3.0 implementations. It is
possible that implementations may support values greater than the
minimum values described. The actual hardware-dependent values
for fragment shader built-in values can also be queried from API
code. The following code shows how you would query the values of
gl_MaxTextureImageUnits
and
gl_MaxFragmentUniformVectors
:
GLint maxTextureImageUnits, maxFragmentUniformVectors;
glGetIntegerv(GL_MAX_TEXTURE_IMAGE_UNITS,
&maxTextureImageUnits);
glGetIntegerv(GL_MAX_FRAGMENT_UNIFORM_VECTORS
&maxFragmentUniformVectors);
Precision qualifiers were briefly introduced in Chapter 5, “OpenGL ES
Shading Language” and were covered in detail in Chapter 8, “Vertex
Shaders.” Please review those sections for full details on precision
qualifiers. We remind you here that there is no default precision for
fragment shaders. As a consequence, every fragment shader must
declare a default precision (or provide precision qualifiers for all variable
declarations).
