CODEWITHSUNDEEP
javaopengllwjglorthographic
user3146918
user3146918

Reputation: 23

What do glOrtho Parameters Correspond To?

I recently started using LWJGL and noticed that the glOrtho method takes the parameters left, right, bottom, top and cannot figure out what these correspond to.

I was confused when I noticed that 

glOrtho(1, 1, 1, 1, 1, -1);

was the same as 

glOrtho(-1, 1, -1, 1, 1, -1);

Am I correct in saying these to snippets of code are the same and if so why?

Upvotes: 2

Views: 998

Answers (2)

ssell
ssell

Reputation: 6597

An orthographic projection is a rectangular parallelepiped (aka a box). The parameters for the glOrtho call define the clipping planes, or edges, of this box. Anything that lies outside of the projection box will not be rendered.

In regards to your example calls, the first one (glOrtho(1,1,1,1,1,-1)) is invalid and generates an GL_INVALID_OPERATION). Why? An orthographic projection is defined as

enter image description here

Notice the division by 0 errors due to right == left and top == bottom.

The second example (glOrtho(-1,1,-1,1,1,-1)) simply creates an identity matrix.

Chapter 3 - OpenGL Programming Guide - Projection Transforms

ScratchPixel - Orthographic Projection Tutorial

Upvotes: 4

derhass
derhass

Reputation: 45362

No, they are not the same:

glOrtho(1, 1, 1, 1, 1, -1);

will just generate a GL_INVALID_OPERATION error, and the commend has no further effect. The error occurs when left==right or top==bottom or near==far. (If it did not check for that condition, a division by zero would occur).

glOrtho(-1, 1, -1, 1, 1, -1);

is effectively doing nothing, as it will multiply the current matrix by an identity matrix.

It might appear as if those commands have the same result, but what actually happens is quite different.

Upvotes: 4

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