Graphics Reference
In-Depth Information
source for that component can be either
GL_RED
,
GL_GREEN
,
GL_BLUE
,
or
GL_ALPHA
to fetch from the R, G, B, or A component, respectively
.
Additionally, the application can set the value to be the constant 0 or 1
using
GL_ZERO
or
GL_ONE
, respectively.
In some applications, it is useful to be able to start displaying a scene
before all of the texture mipmap levels are available. For example, a GPS
application that is downloading texture images over a data connection
might start with the lowest-level mipmaps and display the higher levels
when they become available. In OpenGL ES 3.0, this can be accomplished
by using several of the arguments to
glTexParameter[i|f][v]
. The
GL_TEXTURE_BASE_LEVEL
sets the largest mipmap level that will be used for
a texture. By default, this has a value of 0, but it can be set to a higher value
if mipmap levels are not yet available. Likewise,
GL_TEXTURE_MAX_LEVEL
sets the smallest mipmap level that will be used. By default, it has a value of
1000 (beyond the largest level any texture could have), but it can be set to
a lower number to control the smallest mipmap level to use for a texture.
To select which mipmap level to use for rendering, OpenGL ES
automatically computes a level of detail (LOD) value. This floating-
point value determines which mipmap level to filter from (and in
trilinear filtering, controls how much of each mipmap is used). An
application can also control the minimum and maximum LOD values
with
GL_TEXTURE_MIN_LOD
and
GL_TEXTURE_MAX_LOD
. One reason it
is useful to be able to control the LOD clamp separately from the base
and maximum mipmap levels is to provide smooth transitioning when
new mipmap levels become available. Setting just the texture base and
maximum level might result in a popping artifact when new mipmap
levels are available, whereas interpolating the LOD can make this
transition look smoother.
The last texture parameters to discuss are
GL_TEXTURE_COMPARE_FUNC
and
GL_TEXTURE_COMPARE_MODE
. These texture parameters were introduced
to provide a feature known as percentage closest filtering (PCF). When
performing the shadowing technique known as shadow mapping, the
fragment shader needs to compare the current depth value of a fragment
to the depth value in a depth texture to determine whether a fragment
is within or outside of the shadow. To achieve smoother-looking shadow
edges, it is useful to be able to perform bilinear filtering on the depth
