In OpenGL, can I draw a pixel that exactly at the

Question

By (5, 5) I mean exactly the fifth row and fifth column.

I found it very hard to draw things using screen coordinates, all the coordinates in OpenGL is relative, and usually ranging from -1.0 to 1.0. Why it is so serious to prevent programmers from using screen coordinates / window coordinates?

Answer 1

I did a bit of 3D programming several years back and, while I'm far from an expert, I think you are overlooking a very important difference between classical bitmapped DrawPixel(x, y) graphics and the type of graphics done with Direct3D and OpenGL.

Back in the days before 3D, computer graphics was mostly about bitmaps, which is to say collections of colored dots. These dots had a 1:1 relationship with the pixels on your monitor.

However, that had numerous drawbacks, including making 3D very difficult and requiring bitmaps of different sizes for different display resolutions.

In OpenGL/D3D, you are dealing with vector graphics. Lines are defined by points in a 3-dimensional coordinate space, shapes are defined by lines and so on. Surfaces can have textures, lights can be added, as can various types of lighting effects etc. This entire scene, or a part of it, can then be viewed through a virtual camera.

What you 'see' though this virtual camera is a projection of the scene onto a 2D surface. We're still dealing with vector graphics at this point. However, since computer displays consist of discrete pixels, this vector image has to be rasterized, which transforms the vector into a bitmap with actual pixels.

To summarize, you can't use screen/window coordinates because OpenGL is based on vector graphics.

Answer 2

The simplest way is probably to set the projection to match the pixel dimensions of the rendering space via glOrtho. Then vertices can be in pixel coordinates. The downside is that resizing the window could cause problems and you're mostly wasting the accelerated transforms.

Assuming a window that is 640x480:

// You can reverse the 0,480 arguments depending on you Y-axis 
// direction preference
glOrtho(0, 640, 0, 480, -1, 1);

Frame buffer objects and textures are another avenue but you'll have to create your own rasterization routines (draw line, circle, bitmap, etc). There are problaby libs for this.

Answer 3

You can just change the "camera" to make 3D coordinates match screen coordinates by setting the modelview matrix to identity and the projection to an orthographic projection (see my answer on this question). Then you can just draw a single point primitive at the required screen coordinates.

You can also set the raster position with glWindowPos (which works in screen coordinates, unlike glRasterPos) and then just use glDrawPixels to draw a 1x1 pixel image.

Answer 4

@dandan78 OpenGL is not a Vector Graphics renderer. Is a Rasterizer. And in a more precise way is a Standard described by means of a C language interface. A rasterizer, maps objects represented in 3D coordinated spaces (a car, a tree, a sphere, a dragon) into 2D coordinated spaces (say a plane, your app window or your display), these 2d coordinates belong to a discrete coordinated plane. The counter rendering method of rasterization is Ray Tracing.

Vector graphics is a way to represent by means of mathematical functions a set of curves, lines or similar geometrical primitives, in a nondiscrete way. So Vector graphics is in the "model representation" field rather than "rendering" field.