[PD] GEM mapping units to pixels

hi

I am trying to map GEM units to pixels so that i can position some videos in the GEM window using pixels values. For this I understand that i must match the projection values with the window size. So i am doing

dimen 400 300 perspect 0 400 0 300 1 20

and i render all objects at z position 1

translateXYZ 100 100 1

However I am getting a weird result, the units are far smaller that pixels, so to move a video 100px in the x i need to change it x position by 300 units or something like this. So I am not sure about what i am doing wrong, I am familiar with OpenGL but i never played too much with GEM before so I am still getting into it.

Another small issue is that I would also like to reverse the Y so that upper left is 0,0 and bottom left would be 0,200 for example, not sure about how to change this. I guess it is to do with the gemwin settings but i dont get it right.

thanks

enrike