OpenGL define vertex position in pixels

Question

I\'ve been writing a 2D basic game engine in OpenGL/C++ and learning everything as I go along. I\'m still rather confused about defining vertices and their \"position\". Tha

Answer 1

Firstly be aware that OpenGL not uses standard pixel coordinates. I mean by that for particular resolution, ie. 800x600 you dont have horizontal coordinates in range 0-799 or 1-800 stepped by one. You rather have coordinates ranged from -1 to 1 later send to graphic card rasterizing unit and after that matched to particular resolution.

I ommited one step here - before all that you have an ModelViewProjection matrix (or viewProjection matrix in some simple cases) which before all that will cast coordinates you use to an projection plane. Default use of that is to implement a camera which converts 3D space of world (View for placing an camera into right position and Projection for casting 3d coordinates into screen plane. In ModelViewProjection it's also step of placing a model into right place in world).

Another case (and you can use Projection matrix this way to achieve what you want) is to use these matrixes to convert one range of resolutions to another.

And there's a trick you will need. You should read about modelViewProjection matrix and camera in openGL if you want to go serious. But for now I will tell you that with proper matrix you can just cast your own coordinate system (and ie. use ranges 0-799 horizontaly and 0-599 verticaly) to standarized -1:1 range. That way you will not see that underlying openGL api uses his own -1 to 1 system.

The easiest way to achieve this is glOrtho function. Here's the link to documentation: http://www.opengl.org/sdk/docs/man/xhtml/glOrtho.xml

This is example of proper usage: glMatrixMode (GL_PROJECTION) glLoadIdentity (); glOrtho (0, 800, 600, 0, 0, 1) glMatrixMode (GL_MODELVIEW)

Now you can use own modelView matrix ie. for translation (moving) objects but don't touch your projection example. This code should be executed before any drawing commands. (Can be after initializing opengl in fact if you wont use 3d graphics).

And here's working example: http://nehe.gamedev.net/tutorial/2d_texture_font/18002/

Just draw your figures instead of drawing text. And there is another thing - glPushMatrix and glPopMatrix for choosen matrix (in this example projection matrix) - you wont use that until you combining 3d with 2d rendering.

And you can still use model matrix (ie. for placing tiles somewhere in world) and view matrix (in example for zooming view, or scrolling through world - in this case your world can be larger than resolution and you could crop view by simple translations)

After looking at my answer I see it's a little chaotic but If you confused - just read about Model, View, and Projection matixes and try example with glOrtho. If you're still confused feel free to ask.