In OpenGL fix function programming, can I possibly map different textures on different objects, but that texture in generated from one image only. For e.g. I have 1024 X 1024 image. I have four rectangles in my scene. Now I would want to slice image into 256 X 256 *4 and map these sliced images as textures.
How can I do this. One option is to off course pre-slice the image. But can this be done using glTexSubImage2D or some similar/different API?
Yes, you can use texture coordinates, which indicate which parts of the texture you wish to be mapped onto your object, rather than mapping the whole thing.
Read more: http://www.glprogramming.com/red/chapter09.html#name6
EDIT : Shaders are required to use array textures : http://www.opengl.org/registry/specs/EXT/texture_array.txt. I'll leave the answer as it might still be useful info.
I guess you can use an Array Texture for this : http://www.opengl.org/wiki/Array_Texture
You make a 256 x 2048 texture. When loading the texture you specify a layer size (256 x 256) and layer count (4). Your texture will then be split up in four layers.
You can access the texture using UVs : [x, y, LayerId]
Note that if you want to generate mipmaps, you need to define the number of levels when you allocate the storage with glTexStorage3D.
I think you have three options.
Use different texture coordinates for each different rectangle.
Transform the texture coordinates using glMatrixMode(GL_TEXTURE) and a different matrix between drawing each rectangle.
Create four different OpenGL textures from your original big texture. I don't think OpenGL offers you any help here. You have to either use a paint package to do it (easiest option if you only have to do this a few times), or copy parts of the image into a new buffer before calling glTexImage2D.
I think the first option is the easiest, with the advantage that you don't have to change any state between drawing the rectangles.
