If you think there is a problem with Camera, your test should only involve camera, and not Stage/Viewports.

That said, this is a problem with documentation and variable names. It should really be touchToStageCoordinates, not screenToStageCoordaintes, since its expected touch space input. There are some other inconsistencies that probably should be cleaned up that are the same.

Ok, modified.

public void create() {
      Vector3 tmpvec = new Vector3();
      tmpvec.set(0, 0, 0);

      Camera camera = new OrthographicCamera();
      camera.viewportWidth = 100;
      camera.viewportHeight = 100;
      camera.project(tmpvec);
      camera.unproject(tmpvec);
      System.out.println(tmpvec); // prints (0.0,-0.01999998,0.0)
  }

#5854 is the same same issue, and as I said its a problem with documentation as they arent all screen space coordinates. We do not need two issues to discuss this same problem.

Here is a demo that demonstrates that this is a documentation issue. https://gist.github.com/Tom-Ski/3c71425e6a199ec2677fbac39e457e54

Why are your touch coordinates offset by 1?

Regardless, should a sequence of project and unproject return a different value?

      camera.project(tmpvec);
      camera.unproject(tmpvec);

As an example, if you have a screen of 10x10, bottom left coordinate is 0,0. In touch space, this coordinate is 0,9. Gdx.graphics.getHeight() would return 10 however, we need the offset because we start at 0, not at 1.

Range of touch/screen is from (0 to width-1) since we start at 0.

As per the documentation, yes they should. There are just a few inconsistencies with the language used that makes it confusing. Although the documentation (for Camera at least) does describe the difference in coordinate spaces the functions return and expect, it still calls them both screen space, which is the confusing part. The solution here is to improve the docs.

Ok.
However, this is still unrelated to #5854 which is about stages with viewports with y offsets. Even if the method name were stage.stageToTouchCoordinates, I believe there is still an issue there. I'll comment further in that issue.

OpenGL for reference, specifically gluProject and gluUnProject, compiled and run on Arch Linux/Mesa 3D.

gluProject — map object coordinates to window coordinates
gluUnProject — map window coordinates to object coordinates

The window size is 300x300, and (0,0) is the point at the centre of the screen.

#include <stdio.h>
#include <GL/gl.h>
#include <GL/glut.h>

void displayFunc(void)
{
    GLdouble posX, posY, posZ;
    GLdouble winX, winY, winZ;
    GLdouble resX, resY, resZ;
    GLdouble modelview[16];
    GLdouble projection[16];
    GLint viewport[4];
    glGetDoublev(GL_MODELVIEW_MATRIX, modelview);
    glGetDoublev(GL_PROJECTION_MATRIX, projection);
    glGetIntegerv(GL_VIEWPORT, viewport);
    posX = 0.0;
    posY = 0.0;
    posZ = 0.0;

    gluProject(posX, posY, posZ, modelview, projection, viewport, &winX, &winY, &winZ);
    printf("window: %f %f %f\n", winX, winY, winZ);
    // window: 150.000000 150.000000 0.500000

    gluUnProject(winX, winY, winZ, modelview, projection, viewport, &resX, &resY, &resZ);
    printf("pos: %f %f %f\n", resX, resY, resZ); 
    // pos: 0.000000 0.000000 0.000000
}

int main(int argc, char** argv)
{
    glutInit(&argc, argv);
    glutInitDisplayMode(GLUT_DOUBLE);
    glutInitWindowSize(300, 300);
    glutCreateWindow("Test glProject glUnProject");
    glutDisplayFunc(displayFunc);
    glutMainLoop();
    return 0;
}

My problem iPhone X can’t get it to open I’ve tried everything,boot up tried everything

@mark6412 Don't ask for help on an issue tracker. See: https://github.com/libgdx/libgdx/wiki/Getting-help

Seems a bit unfortunate to need to think about touch space coordinates. Ideally we'd have only screen space and world space.