Graphics Reference
In-Depth Information
Table 5-7
Uniform Storage with Packing
Location
X
Y
Z
W
0
m[0].x
m[0].y
m[0].z
f[0]
m[l].x
m[l].y
m[l].z
f[l]
1
m[2].x
m[2].y
m[2].z
f[2]
2
v.x
v.y
v.z
f[3]
3
4
—
—
—
f[4]
5
—
—
—
f[5]
With the packing rules, only six physical constant locations need to be
used. You will notice that the array
f
needs to keep its elements spanning
across row boundaries. The reason for this is that typically GPUs index the
constant store by vector location index. The packing must keep the arrays
spanning across row boundaries so that indexing will still work.
All of the packing that is done is completely transparent to the user of the
OpenGL ES Shading Language, except for one detail: The packing impacts
the way in which uniforms and vertex shader outputs/fragment shader
inputs are counted. If you want to write shaders that are guaranteed to
run on all implementations of OpenGL ES 3.0, you should not use more
uniforms or interpolators than would exceed the minimum allowed
storage sizes after packing. For this reason, it's important to be aware of
packing so that you can write portable shaders that will not exceed the
minimum allowed storage on any implementation of OpenGL ES 3.0.
Precision qualifiers enable the shader author to specify the precision with
which computations for a shader variable are performed. Variables can be
declared to have either low, medium, or high precision. These qualifiers
are used as hints to the compiler to allow it to perform computations
with variables at a potentially lower range and precision. It is possible
that at lower precisions, some implementations of OpenGL ES might
be able to run the shaders either faster or with better power efficiency.
