Q: how to draw a 2D square in a 3D space?

OpenGL OpenGL: Advanced Coding
1

Hi,

I want to draw a 2-D square. It is positioned at a specified 3D location but its shape on the user’s screen should be always a square with the same size regardless of translation, rotation, etc. How can I do this? Could someone help me? Thanks.

Regards.

Tony

2

Get the modelview matrix using glGetDoublev
Fill first 3X3 square in this matrix by
100
010
001
to eliminate rotation.
Extract depth from bottom row of matrix.
Calculate width like w = z*w1/z1 , where z is current depth, w1 is a width desired on z1 depth(this koefficient can be found using some trig).

Hope that helped.

Randy

3

This is my old particles code, hope it helps…

matrix_t modelView;
Vector3 x;
Vector3 a;
Vector3 b;
Vector3 c;

glGetFloatv(GL_MODELVIEW_MATRIX, modelView);

x.x = modelView[0];
x.y = modelView[4];
x.z = modelView[8];

y.x = modelView[1];
y.y = modelView[5];
y.z = modelView[9];

size = parts[i].GetSize();

a = parts[i].GetPos() + ((-x - y) * size);
b = parts[i].GetPos() + ((x - y) * size);
c = parts[i].GetPos() + ((x + y) * size);

//vertex 1
vertici[i*3].x = a.x;
vertici[i*3].y = a.y;
vertici[i*3].z = a.z;
//vertex 2
vertici[(i3)+1].x = b.x;
vertici[(i3)+1].y = b.y;
vertici[(i3)+1].z = b.z;
//vertex 3
vertici[(i3)+2].x = c.x;
vertici[(i3)+2].y = c.y;
vertici[(i3)+2].z = c.z;

Hope it helps

rIO http://www.spinningkids.org/umine

4

Thanks Randy and rIO for your help. The messages surely give me enough lead to solve my problem. Your kindness and time are very appreciated.

Tony