How i can draw projection line from camera to 3D area?

Question

I need a line that would be part of the user interface, but it always pointed to a specific place in 3D space.

To do this, I try:

            double camY, camX;
            camX = cameraX * -1;
            if(camX > 90)
                camX = 180 - camX;
            if(camX < -90)
                camX = -180 - camX;

            camY = cameraY;
            double camY2 = camY;
            if(camY > 90)
                camY2 = 180 - camY;
            if(camY < -90)
                camY2 = -180 - camY;

            double x1;
            double y1;
            double x2 = x * (90.0 - Math.abs(camY)) / 90.0,
            y2 = (y * (90.0 - Math.abs(camY)) / 90.0);
            if(vertical) {
                x1 = x2 + (y * (camY2 / 90.0) * ((90 - camX) / 90));
                y1 = (y2 * ((90 - camX) / 90)) - (x * (camY2 / 90.0));
            }else{
                x1 =  x2 + (y * (camY2 / 90.0));
                y1 = y2 - (x * (camY2 / 90.0));
                y1 = y1 * (camX / 90.0);
            }
            GL11.glVertex2d(x1, y1);
            GL11.glVertex2d(toX, toY);
            GL11.glVertex2d(toX, toY);
            GL11.glVertex2d(max, toY);

Where x and y - coordinate of point on 3D space. cameraX and cameraY - angle of camera rotate. toX`` and toY``` destination point on the camera plane (user interface).

All this code runs before the camera (

        GL11.glOrtho(-max, max, -1, 1, 10, -10);
        glRotatef(cameraX, 1f, 0f, 0);
        glRotatef(cameraY, 0f, 1f, 0); 

) and before GL11.glMatrixMode(GL11.GL_MODELVIEW); . Therefore, it ignores the Z coordinate.

Initially, I had only the last 4 lines of code, but then when the camera was rotated, the entire line moved behind it. So I added the rest of the calculations.

This partially solves my problem.

Initial position:

Camera rotation on the y axis:

Small line deviations are already visible.

Camera rotation on the x and y axis:

As you can see in the screenshots, the red line still shifts when the camera rotates. How can I make it always be at the point I need in spaces? (In the center of the red circle in the screenshots)

Answer 1

You should:

1. Project the 3D point you want the line to end to the screen manually. For that

   • Get the model-view and projection matrices with glGetFloatv , GL_MODELVIEW_MATRIX and GL_PROJECTION_MATRIX .
   • Multiply your 3d point by the matrices, perform a perspective division, and convert to your viewport coordinates.

2. Draw a 2d line from the UI location you want it to begin to the projected 2d location you want it to end.