Texturing - Praise for OpenGL ES 30 Programming Guide - page 267

This function will update the region of texels in the range ( xoffset , yoffset )
to ( xoffset + width - 1, yoffset + height - 1). Note that to use this function,
the texture must already be fully specified. The range of the subimage
must be within the bounds of the previously specified texture image. The
data in the pixels array must be aligned to the alignment that is specified
by GL_UNPACK_ALIGNMENT with glPixelStorei .

There is also a function for updating a subregion of a compressed 2D
texture image—that is, glCompressedTexSubImage2D . The definition for
this function is more or less the same as that for glTexImage2D .

void glCompressedTexSubImage2D (GLenum target ,
GLint level , GLint xoffset ,
GLint yoffset , GLsizei width ,
GLsizei height ,
GLenum format ,
GLenum imageSize ,
const void* pixels )

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)

target

specifies which mip level to update

level

the x index of the texel to start updating from

xoffset

the y index of the texel to start updating from

yoffset

the width of the subregion of the image to update

width

the height of the subregion of the image to update

height

the compressed texture format to use; must be
the format with which the image was originally specified

format

contains the actual pixel data for the subregion of the image

pixels

In addition, as with 2D textures, it is possible to update just a subregion of
an existing 3D texture and 2D texture arrays using glTexSubImage3D .

void glTexSubImage3D (GLenum target , GLint level ,

GLint xoffset , GLint yoffset ,
GLint zoffset , GLsizei width ,
GLsizei height , GLsizei depth ,
GLenum format , GLenum type ,
const void* pixels )

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