How do I convert a 3D point to screen coordinates?

[nogix]

I'm trying to make a mod that renders entity health bars in an external window (same width and height as the game window), not the main game window.

To do this, I need to convert an entity's position (x, y, z) to 2D screen coordinates (x, y) so I can render the health bar on the external window.

The external window does not use LWJGL so I cannot use any LWJGL commands to render the health bar.

How would I go about doing this? I tried the method used here, however the 2D coordinates were very incorrect.

Edited September 1, 2019 by nogix

[Animefan8888]

  On 9/1/2019 at 5:52 PM, nogix said:

How would I go about doing this?

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Is this like a map kinda thing?

[nogix]

  On 9/1/2019 at 6:43 PM, Animefan8888 said:

Is this like a map kinda thing?

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Nope

[Animefan8888]

  On 9/1/2019 at 6:52 PM, nogix said:

Nope

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Why dont you explain more on what you are trying to do. Because as it stands right now I'm not sure what you mean by translate 3d to 2d and then

 

  On 9/1/2019 at 5:52 PM, nogix said:

renders entity health bars in an external window (same width and height

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[nogix]

  On 9/1/2019 at 7:04 PM, Animefan8888 said:

Why dont you explain more on what you are trying to do. Because as it stands right now I'm not sure what you mean by translate 3d to 2d and then

 

 

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Basically, imagine taking the player's name tag (the thing that renders above a player's head in multiplayer) and rendering just that (nothing else) at the same position onto an external window that is not Minecraft.

In my case, I'm not trying to render the name tag, I'm trying to render a health bar. This would be trivial to do in the actual Minecraft window since it uses LWJGL (which does not require you to calculate screen coordinates), but since my external window is a JFrame and uses Java's Graphics2D, I cannot render the health bar without having the screen coordinates. What I'm referring to by "translate 3d to 2d" is world to screen projection.

Edited September 1, 2019 by nogix

[Animefan8888]

  On 9/1/2019 at 7:07 PM, nogix said:

same position onto an external window

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This is the part that doesnt make sense. I guess you can just subtract the client players x,z values from the entities x,z. And of course scale them according to the amount of chunks the player has rendered and the size of your JFrame.

[DavidM]

I suppose you are trying to render the health bar at exactly where the mobs are at on the original Minecraft screen, in which case you would need to offset the local position by the camera's rotation.

I don't know if there is a GLM in Java or not, but if there is not, you can use the following method.

 

To transform a 3D pos to 2D includes the following states:

1. 3D original (global) entity position
2. 3D relative (to camera) entity position
3. 3D relative (to camera) entity position after camera rotation
4. 2D screen coords

To elaborate the concept better, I will assume the center of the window is (0, 0). Left side is negative x, right side is positive x, etc. This is not the case of a JFrame though, and you will have to do some manual translation of the screen coords by adding half the screen width to x, and half the screen height to y.

 

First, subtract every axis (x, y, z) of the entity's global position by the camera's global position (the position in the Minecraft world). This will give a relative position from the camera to the entity.

 

Next, apply the camera's rotation to the relative position. Assuming the Z-axis is the one sticking out from the camera on the local coordinates (I am not sure if this is the case of Minecraft; change this to X if necessary), multiply z by cos(camera.yaw), and multiply x by sin(camera.yaw). This takes care of the yaw (horizontal) rotation of the camera. The same applies to the pitch (vertical) rotation of the camera; simply multiply z (again) by cos(camera.pitch) and multiply y by sin(camera.pitch).

 

You are basically done now if you want orthogonal view. Throw away the z and use the x and y as screen coords. However, I assume that you want perspective view, so you need to make x = x / z * FOV and y = y / z * FOV, where FOV is the field of view value you need to experiment with (start with something near 2).

 

Now just throw away the z (or the x, depending on which axis is the one sticking out the camera in Minecraft when the camera has no rotation) and use x, y as the screen coords. Remember to do the manual translation of the screen coords by adding half the screen width to x, and half the screen height to y.

Edited September 2, 2019 by DavidM