Question

I’m creating an application in OpenGL in which I’m using glOrtho() with origin in top left corner:

glOrtho(0.0, width, height, 0.0, 0.0, 1.0);

I haven’t found it was wrong until I tried to render outline font, which appeared upside-down. After searching information about it, I came across site on which I read that my origin should start from bottom left corner.

I knew OpenGL is using bottom left corner origin, but I found it easier for me to use top left one. I just had to flip all textures horizontally and everything was ok. But yes, the problem appeared with those outline fonts which are appearing upside-down.

Now to change origin place, I’d have to change a bunch of code so if it’s not necessary, I’d really like to avoid this.

So my question is:
Is my approach with top left origin very wrong? And if not, is there any way to make those outline fonts appear properly?

Answer 1

You are rendering that font somewhere, right? How are you rendering it, and what is stopping you from applying a vertical mirror transform matrix at that time?

I.e. if you are doing this the old-fashioned, obsolete way:

glScalef( 1, -1, 1 );
glCallList(font); // Or whatever you are doing to render the outline font

Or if you are using shaders, have your vertex Shader do something like this:

  Vec4 position = aPosition;
  position.y = -1 * position.y;
  gl_Position = aMVPMatrix * position

This all being pseudo-code, of course.