Stage.stageToScreenCoordinates returns incorrect value with y offset in viewport

[jusw85]

Issue details

Stage.stageToScreenCoordinates returns incorrect value with y offset in viewport

Reproduction steps/code

Create a 100x100 window.
Setup a stage with a viewport in the top quarter of the window i.e. width 100, height 25, y=75
stage.stageToScreenCoordinates returns incorrect values.

public void create() {
     Viewport viewport = new Viewport() {
         @Override
         public void update(int screenWidth, int screenHeight, boolean centerCamera) {
             assert screenWidth == 100 && screenHeight == 100;
             setScreenBounds(0, 75, 100, 25);
             setWorldSize(100, 25);
             apply(centerCamera);
         }
     };
     viewport.setCamera(new OrthographicCamera());
     Stage stage = new Stage(viewport);

     Vector2 tmpvec = new Vector2();

     tmpvec.set(0, 0);
     stage.getViewport().project(tmpvec);
     System.out.println(tmpvec); // prints (0, 75), correct so far

     tmpvec.set(0, 0);
     stage.stageToScreenCoordinates(tmpvec);
     System.out.println(tmpvec); // prints (0, -50), should be (0, 25)
 }

Due to the following line in Stage:

libgdx/gdx/src/com/badlogic/gdx/scenes/scene2d/Stage.java

Line 738 in c0c1f51

stageCoords.y = viewport.getScreenHeight() - stageCoords.y;

After getting the result of camera.project (in absolute screen coordinates with origin at bottom left), it converts to top left origin using viewport.screenHeight instead of actual window height.

Version of LibGDX and/or relevant dependencies

1.9.10

[jusw85]

This is unrelated to the #5853

[jusw85]

With reference to the example code:

     tmpvec.set(0, 0);
     stage.stageToScreenCoordinates(tmpvec);
     System.out.println(tmpvec);

(0,0) is within the bounds of the window. Why is it returning (0,-50) which is outside the window?

[Tom-Ski]

Yeah, this doesn't look right. Why do you expect 0,25 though?

[jusw85]

1. The window has width=100, height=100.
2. The viewport has screenWidth=100, screenHeight=25, screenX=0, screenY=75 i.e. top 1/4 of the window.
3. (0, 0) in world coordinates should correspond to (0, 75) in screen coordinates (origin at bottom left). This is verified by stage.getViewport().project(tmpvec);. This calls camera.project.

Relevant camera.project documentation:

The screen coordinate system has its origin in the <b>bottom</b> left, with the y-axis pointing <b>upwards</b> and the x-axis pointing to the right.

Relevant code:

libgdx/gdx/src/com/badlogic/gdx/utils/viewport/Viewport.java

Line 82 in c0c1f51

public Vector2 project (Vector2 worldCoords) {

libgdx/gdx/src/com/badlogic/gdx/graphics/Camera.java

Line 239 in 2161c43

public Vector3 project (Vector3 worldCoords, float viewportX, float viewportY, float viewportWidth, float viewportHeight) {

4. stage.stageToScreenCoordinates has origin at top left. Therefore it should be (0,25) (or thereabouts), certainly not (0,-50).

stageToScreenCoordinates code, which uses viewport.screenHeight to convert the origin:

libgdx/gdx/src/com/badlogic/gdx/scenes/scene2d/Stage.java

Line 736 in c0c1f51

public Vector2 stageToScreenCoordinates (Vector2 stageCoords) {

[Tom-Ski]

stage.stageToScreenCoordinates shouldn't have origin at top left, so it shouldn't be 0,25. Screen is bottom left unless otherwise specified, and project is sticking to this standard. As with the previous issue, the documentation probably needs improving here also.

Anyway, this result seems wrong, so will have to do further testing.

[jusw85]

stage.stageToScreenCoordinates shouldn't have origin at top left

I reasoned this from the way the source code and actual tests work, specifically this line stageCoords.y = viewport.getScreenHeight() - stageCoords.y;.

It works perfectly fine, returning correct result with origin top left if viewport has no offset.

[edit] actually origin top left is used almost everywhere:

viewport.toScreenCoordinates (origin top left)

/** Transforms a point to real screen coordinates (as opposed to OpenGL ES window coordinates), where the origin is in the top
* left and the the y-axis is pointing downwards. */

camera.unproject (origin top left)

The x- and y-coordinate of vec are assumed to be in screen coordinates (origin is the top left corner, y pointing down, x pointing to the right)

[NathanSweet]

Didn't mean for this to get closed (happened via GH's automatic magicke).