Java LWJGL OpenGL convert 3d point to 2d point

问题

I'm trying to convert a 3d point in OpenGL to a 2d point on screen to render a healthbar for a little game I'm writing. However, I'm having some trouble retrieving the x coordinate of where to draw the healthbar. Basically, the healthbar must appear to be above a player, but must always have the same width/height relative to the screen.

I tweaked a snippet of code I found from the accepted answer at Convert a 3D location to a 2D on-screen point. (XYZ => XY) and I now have this

public static int[] getScreenCoords(double x, double y, double z) {
    FloatBuffer screenCoords = BufferUtils.createFloatBuffer(4);
    IntBuffer viewport = BufferUtils.createIntBuffer(16);
    FloatBuffer modelView = BufferUtils.createFloatBuffer(16);
    FloatBuffer projection = BufferUtils.createFloatBuffer(16);
    // int[] screenCoords = new double[4];
    // int[] viewport = new int[4];
    // double[] modelView = new double[16];
    // double[] projection = new double[16];
    GL11.glGetFloat(GL11.GL_MODELVIEW_MATRIX, modelView);
    GL11.glGetFloat(GL11.GL_PROJECTION_MATRIX, projection);
    GL11.glGetInteger(GL11.GL_VIEWPORT, viewport);
    boolean result = GLU.gluProject((float) x, (float) y, (float) z, modelView, projection, viewport, screenCoords);
    if (result) {
        return new int[] { (int) screenCoords.get(3), (int) screenCoords.get(1) };
    }
    return null;
}

It seems to work fine with the y coordinate, however, x always returns 0 no matter what the angle is.

Many thanks in advance!

回答1:

screenCoords.get(3) should be screenCoords.get(0) because the x position is stored at index 0. You also only actually need the capacity of screenCoords to be 3 floats, not 4.

来源：https://stackoverflow.com/questions/21950105/java-lwjgl-opengl-convert-3d-point-to-2d-point