In opengl, How can I get relation between pixels and gl.gltranslatef(floatx,y,z)?

Question

Hi all,

I am trying to learn opengl stuff on Android. In the gl.gltranslatef(x,y,z) call, I am shifting my texture by some units in the +ve x direction. But I am unable to find the number of pixels does 1 unit of x belong to?

Here is what I am doing: I call gl.glviewport(0,0,width,height); // This will set my rectangle with 0,0 as lowerleft corner and then extend it to accommodate width and height.

Then I call to gl.glfrustrum(-5,5,-7,7,3,7); // I am little confused how this call is using the dimensions I set in gl.glviewport.

How will -5 to 5 units from left to right in the above call, translate to pixels on the screen of android?

I mean if width = 320 and height = 533 pixels, then what will be the number of pixels occupied on the screen due to the gl.glfrustrum call?

I am experimenting in the gl.gltranslatef call by specifying xshift as 5.0, but it does not translate the bitmap at the right or left corner of the screen, when I increase it to 6, part of it is still visible on the screen.

Thanks Siddhesh

In short, I am searching for the maximum number of units (in terms of X) which will represent extreme corners of my android phone screen.

Answer 1

glViewpoint tells it what rectangle (in pixels) your OpenGL output should be displayed in.

glFrustum tells it what coordinates in your "world" units should be mapped to that viewport.

An important point: your glFrustum call includes not only a height and width, but also a depth. Since you are specifying a Frustum, not a cube, that means anything with a Z coordinate anywhere but the very front of your frustum will be scaled down appropriately for its distance from the viewer.

As such, when you to a glTranslatef, the distance by which a particular object will move (in terms of pixels) will depend on its distance from the viewer. The further away it is from the viewer, the fewer pixels a particular sideways or up/down will translate to.

Depending on what else you're doing, one easy way to deal with this might be to use glOrtho instead of glFrustum. glOrtho gives orthographic mode, which means no perspective scaling is done, so a given X or Y distance will translate to the same number of pixels, regardless of distance from the viewer.