OpenGL Perspective Projection pixel perfect drawing

Question

The target is to draw a shape, lets say a triangle, pixel-perfect (vertices shall be specified in pixels) and be able to transform it in the 3rd dimension.

I've tried it with a orthogonal projection matrix and everything works fine, but the shape doesn't have any depth - if I rotate it around the Y axis it looks like I would just scale it around the X axis. (because a orthogonal projection obviously behaves like this). Now I want to try it with a perspective projection. But with this projection, the coordinate system changes completely, and due to this I can't specify my triangles verticies with pixels. Also if the size of my window changes, the size of shape changes too (because of the changed coordinate system).

Is there any way to change the coordinate system of the perspective projection so that I can specify my vertices like if I would use the orthogonal projection? Or do anyone have a Idea how to achieve the target described in the first sentence?

The two requirements are inherently impossible to achieve both at the same time, because the perspective transform involves a division step which can result in non-integer coordinates. The best you can hope for is to have one "z plane" where vertex coordinates translate 1:1 into pixels, but in front of or behind that plane, you will still end up with non-integers. — Thomas
What would be the best way to achieve this "z-plane"? I think this is exactly what I want. — Met
The z value for that "pixel plane" will depend on the FOV of your projection. It is easy to find with simple triangle geometry. — derhass

Rabbid76 Rabbid76 · Accepted Answer · 2017-09-18T18:06:12

The projection matrix describes the mapping from 3D points of a scene, to 2D points of the viewport. It transforms from eye space to the clip space, and the coordinates in the clip space are transformed to the normalized device coordinates (NDC) by dividing with the w component of the clip coordinates. The NDC are in range (-1,-1,-1) to (1,1,1).

At Perspective Projection the projection matrix describes the mapping from 3D points in the world as they are seen from of a pinhole camera, to 2D points of the viewport.
The eye space coordinates in the camera frustum (a truncated pyramid) are mapped to a cube (the normalized device coordinates).

Perspective Projection Matrix:

r = right, l = left, b = bottom, t = top, n = near, f = far

2*n/(r-l)      0              0               0
0              2*n/(t-b)      0               0
(r+l)/(r-l)    (t+b)/(t-b)    -(f+n)/(f-n)    -1    
0              0              -2*f*n/(f-n)    0

where:

aspect = w / h
tanFov = tan( fov_y * 0.5 );

prjMat[0][0] = 2*n/(r-l) = 1.0 / (tanFov * aspect)
prjMat[1][1] = 2*n/(t-b) = 1.0 / tanFov

I assume that the view matrix is the identity matrix, and thus the view space coordinates are equal to the world coordinates.
If you want to draw a polygon, where the vertex coordinates are translated 1:1 into pixels, then you have to draw the polygon in parallel plane to the viewport. This means all points have to be draw with the same depth.
The depth has to choose that way, that the transformation of a point in normalized device coordinates, by the inverse projection matrix gives the vertex coordinates in pixel. Note, the homogeneous coordinates given by the transformation with the inverse projection matrix, have to be divided by the w component of the homogeneous coordinates, to get cartesian coordinates.
This means, that the depth of the plane depends on the field of view angle of the projection:

Assuming you set up a perspective projection like this:

float vp_w  = ....  // width of the viewport in pixel
float vp_h  = ....  // height of the viewport in pixel
float fov_y = ..... // field of view angle (y axis) of the view port in degrees < 180°

gluPerspective( fov_y, vp_w / vp_h, 1.0, vp_h*2.0f );

Then the depthZ of the plane with a 1:1 relation of vertex coordinates and pixels, will be calculated like this:

float angRad = fov_y * PI / 180.0;
float depthZ = -vp_h / (2.0 * tan( angRad / 2.0 ));

Note, the center point of the projection to the view port is (0,0), so the bottom left corner point of the plane is (-vp_w/2, -vp_h/2, depthZ) and the top right corner point is (vp_w/2, vp_h/2, depthZ). Ensure, that the near plane of the perspective projetion is less than -depthZ and the far plane is greater than -depthZ.

See further:

OpenGL Perspective Projection pixel perfect drawing

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