8372761: Prevent degenerate transforms for zero-size Scene/SubScene

[mstr2]

When a Scene or SubScene has its width or height set to 0, coordinate transformations stop working because the transformation matrix contains NaNs. This is a consequence of divisions by zero in both parallel and perspective projections.

Even though a 0x0 scene is mathematically degenerate, it is still desirable for coordinate transformation APIs to remain numerically well-behaved and return finite results whenever possible, rather than being poisoned by NaN/Infinity.

Here is an application that demonstrates the failing coordinate transformations. Click on the buttons to resize the SubScene and observe the console output. After applying the patch in this PR, you should observe that localToScreen() no longer returns NaN.

public class ZeroSizeSubScene extends Application {

    @Override
    public void start(Stage stage) {
        var rect = new Rectangle(50, 50, Color.RED);
        var subScene = new SubScene(new Group(rect), 100, 100);
        subScene.setFill(Color.GRAY);

        // Also try a perspective camera:
        //
        // var camera = new PerspectiveCamera();
        // camera.setRotate(45);
        // camera.setTranslateX(-20);
        // camera.setRotationAxis(new Point3D(0, 1, 0));
        // subScene.setCamera(camera);

        class MyButton extends Button {
            public MyButton(String text, double width, double height) {
                super(text);
                setOnAction(_ -> {
                    var timeline = new Timeline(
                        new KeyFrame(Duration.seconds(1), new KeyValue(subScene.widthProperty(), width)),
                        new KeyFrame(Duration.seconds(1), new KeyValue(subScene.heightProperty(), height)));

                    timeline.setOnFinished(_ -> {
                        Point2D p0 = rect.localToScreen(0, 0);
                        System.out.println("rect.localToScreen(0, 0) = " + p0);
                    });

                    timeline.play();
                });
            }
        }

        VBox.setMargin(subScene, new Insets(0, 0, 20, 0));

        var root = new VBox(5,
            subScene,
            new CheckBox("Rotate SubScene") {{ setOnAction(_ -> subScene.setRotate(isSelected() ? 45 : 0)); }},
            new MyButton("Size: 100x100", 100, 100),
            new MyButton("Size: 10x10", 10, 10),
            new MyButton("Size: 100x0", 100, 0),
            new MyButton("Size: 0x100", 0, 100),
            new MyButton("Size: 0x0", 0, 0)
        );

        Scene scene = new Scene(root, 300, 300);
        stage.setScene(scene);
        stage.show();
    }
}

---

Progress

• Change must not contain extraneous whitespace
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Issue

• JDK-8372761: Prevent degenerate transforms for zero-size Scene/SubScene (Bug - P4)

Reviewers

• Andy Goryachev (@andy-goryachev-oracle - Reviewer)

Reviewing

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[bridgekeeper]

👋 Welcome back mstrauss! A progress list of the required criteria for merging this PR into master will be added to the body of your pull request. There are additional pull request commands available for use with this pull request.

[openjdk]

❗ This change is not yet ready to be integrated.
See the Progress checklist in the description for automated requirements.

[mstr2]

/reviewers 2

[openjdk]

@mstr2
The total number of required reviews for this PR (including the jcheck configuration and the last /reviewers command) is now set to 2 (with at least 1 Reviewer, 1 Author).

[mlbridge]

Webrevs

• 01: Full - Incremental (f7c7a3fd)
• 00: Full (3ca281f5)

[kevinrushforth]

I took a quick look and it seems reasonable. I'll let others formally review it, though.

Reviewers: @arapte @lukostyra

/reviewers 2

[openjdk]

@kevinrushforth
The total number of required reviews for this PR (including the jcheck configuration and the last /reviewers command) is now set to 2 (with at least 1 Reviewer, 1 Author).

[andy-goryachev-oracle]

maybe we could declare a static constant EPSILON and use it at least in the modified tests?

[mstr2]

Since there are many different epsilons used in this test class, I'd have to call it something specific like EPSILON_0005, and at that point, there's little difference between a numeric literal and a constant field. It's a bit like saying "ONE" instead of "1".

[andy-goryachev-oracle]

minor suggestion: create a private static function (safeSize ?) with the comment explaining why.

[andy-goryachev-oracle]

Looks good on macOS.

[andy-goryachev-oracle]

@lukostyra could you please take a look at this on Windows with fractional scale?