Access Texture data without texture on shaders


I have a small issue.
I have a texture on a shader, which I can use doing:

vec4 temp = texture2DRect(iText, pxy + offset[i]);

Then I do some processing and the result will be:

oI1 += temp*var1;

oI1 is a vec4 , that contains rgba values. Now I need another processing, where I need to access those rgba values according to some coordinates similar to a texture mapping. The question is: how can you access those rgb values with those coordinates from oI1?