(C++) Camera::Project - Weird Results When Looking Away From Project Point

[T0X1N]

I am trying to use Camera::Project and Context::DrawLine to draw a 2d grid in 3d space for an map editor I am working on, but if 1 of the points go behind the camera, the grid gets goofy and is not drawing the line to connect to where it should in 3D space. Is this a bug or am I doing something wrong?

 

I see that the "Z" axis goes to a negative number when the point is behind the camera, but I don't understand why the X & Y axis go way out of what they are supposed to be nor how to get them to what they are supposed to be.

 

 

I have written a simple code snippet that does the same issue:

 

Vec3 p1 = Vec3(context->GetWidth()/2, context->GetHeight()/2, 0);
Vec3 p2 = camera->Project(Vec3(0, 0, 10));

context->DrawLine(p1.x, p1.y, p2.x, p2.y);

 

Here is a screenshot of what I am experiencing with the code above:

 

[martyj]

It could be due to something like this.

http://i.stack.imgur.com/RXJ3d.jpg

 

Try specifying a different z-axis for your p1.

 

Notice how the camera is at p(0, 0, 0), but the image placement is forward in the z direction.

[T0X1N]

Thanks for the reply, Maryj. Odd thing is it does not matter what p1 nor what p2 is on the 3D space. I tried p1 as a 3D point on the grid (-10 Z units away from p2) and used Camera::Project to grab the 2D coordinates so I can draw the line, but same issue.

 

I think I know what the issue is, but I don't know the solution. I have the scene setup that you can fly the camera around using WASD and mouse to look. If one of the 3D points, of which I am using Camera::Project to get 2D coordinates to draw the line, go behind the camera, the Camera::Project basically returns a mirrored value and a negative Z axis. I am no math guru and I don't know of a formula to mirror those values back to where they should be so the program can draw the line correctly.

 

The end goal here is to draw a 2D grid in the 3D space, just like how the Leadwerks Editor has. I could use a large 3d plane with a grid texture to make the grid, but I would rather do it with 2D lines so I don't use any polygons nor material loaded in memory to draw it.

[f13rce]

I remember this problem when using Project to draw health bars. I solved it by doing a dot product to see whether or not a point is in front of the camera or not.

 

I don't have the code right now but can give it to you in about 12 hours from now if you want to.

 

Edit: Oops, saw too late that you still want to draw the line. I don't have a solution for that though. Although you can use the dot product to determine whether or not it should be drawn

[T0X1N]

Thanks for replying Evayr! That issue would be easy to solve since if the camera is not looking at the point, it would return a -Z value, so I would just stop drawing it if it was just one point.

 

However, the problem here is I want to draw a line of two points. If one point goes off camera, I have want to keep drawing the line, same would be if both points are off screen, but I am looking at the path.

 

Like I said in my previous post, I think the problem is that when the Z axis goes negative, the X & Y coordinates are mirrored on the screen. I just need to mirror them back and BINGO! I should have it fixed.

 

I'll keep working at this and see if I can fix it. If anybody has a solution before me, please chime in!