so if i multiple real coordinates (i mean the one we do with gltranslatef()) with gl_modelviewmatrix[…]
Stop right there. glTranslatef() has nothing to do with vertices or coordinates. glTranslatef() constructs a 4x4 translation matrix T and and automatically multiplies the current (or top) matrix, on the currently selected stack, e.g. GL_MODELVIEW, by this translation matrix: NewMatrix = T * C
Now, gl_ModelViewMatrix will contain whatever matrix is currently at the top of the GL_MODELVIEW matrix stack. If you do the following
vec4 EyeSpacePos = gl_ModelViewMatrix * gl_Vertex;
you can see the obvious result of that operation.
once everything is in the same space, i can freely compute them with each other ?
You can compute whatever you want - GLSL won’t hinder you. Computing stuff with operands that are in the same space, however, can produce meaningful results in the current context. Calculating stuff with operands from unrelated spaces most likely won’t.
by the way, is gl_vertex in glsl real coordinates without any multiplication ?
gl_Vertex is set to the value the current invocation of the vertex shader will process, i.e. if you send some vertices v1, v2, v3, … and so to the GPU, no matter how, the vertex shader will be invoked multiple times to process all vertices. Invocation one may process v1, invocation 2 may process v2 and so on. gl_Vertex will take on the value of v_n for some invocation m. In general, vertices are defined to be in object-space - that’s why we multiply be the model-view matrix in the first place. To take them to a space where other vertices from objects may already be.
and i have heard that -gl_position = eye vector in glsl
This might be the case if you somehow manage to set gl_Position = -eye. I may be ignorant, but I’ve not seen that as of yet.
, how true is that ?
In general such a suggestion is complete nonsense.
One final thought: I think you need to work through an introductory text on general graphics programming and a similar text on OpenGL. You get many important things mixed up and reading this forum alone will not save you when problems come up. And come up they will.