Graphics Reference
In-Depth Information
Another feature introduced in OpenGL ES 3.0 to help improve application
performance is
immutable textures
. As discussed earlier in this chapter, an
application specifies each mipmap level of a texture independently using
functions such as
glTexImage2D
and
glTexImage3D
. The problem this
creates for the OpenGL ES driver is that it cannot determine until draw time
whether a texture has been fully specified. That is, it has to check whether
each mipmap level or subimage has matching formats, whether each level has
the correct dimensions, and whether there is sufficient memory. This draw
time check can be costly and can be avoided by using immutable textures.
The idea behind immutable textures is simple: The application specifies the
format and size of a texture before loading it with data. In doing so, the
texture format becomes immutable and the OpenGL ES driver can perform
all consistency and memory checks up-front. Once a texture has become
immutable, its format and dimensions cannot change. However, the
application can still load it with image data by using
glTexSubImage2D
,
glTexSubImage3D
, or
glGenerateMipMap
, or by rendering to the texture.
To create an immutable texture, an application would bind the texture
using
glBindTexture
and then allocate its immutable storage using
glTexStorage2D
or
glTexStorage3D
.
void
glTexStorage2D
(GLenum
target,
GLsizei
levels,
GLenum
internalFormat,
GLsizei
width,
GLsizei
height
)
void
glTexStorage3D
(GLenum
target,
GLsizei
levels,
GLenum
internalFormat,
GLsizei
width,
GLsizei
height,
GLsizei
depth
)
target
specifies the texture target, either
GL_TEXTURE_2D
or
one of the cubemap face targets
(GL_TEXTURE_CUBE_MAP_POSITIVE_X
,
GL_TEXTURE_CUBE_MAP_NEGATIVE_X, and so on
) for
glTexStorage2D
, or
GL_TEXTURE_3D
or
GL_TEXTURE_2D_ARRAY
for
glTexStorage3D
levels
specifies the number of mipmap levels
internalFormat
the sized internal format for the texture storage;
the full list of valid
internalFormat
values is the
same as the valid sized
internalFormat
values for
glTexImage2D
provided in the Texture Objects and
Loading Textures section earlier in this chapter.
