java - libgdx drawing arc curve

Question

The arc function of libgdx instead of drawing a arc draws a pie segment (ie. has 2 lines connecting to the arc's origin)

shapeRenderer.begin(ShapeType.Line);
shapeRenderer.arc(x, y, radius, 30, 120);
shapeRenderer.end();

Is there a solution to this problem so that libgdx can draw an arc curve similar to the html5 canvas arc function?

Answer 1

Reading the source code, this seems built-in behavior:

/** Draws an arc using {@link ShapeType#Line} or {@link ShapeType#Filled}. */
public void arc (float x, float y, float radius, float start, float degrees, int segments) {
    if (segments <= 0) throw new IllegalArgumentException("segments must be > 0.");
    float colorBits = color.toFloatBits();
    float theta = (2 * MathUtils.PI * (degrees / 360.0f)) / segments;
    float cos = MathUtils.cos(theta);
    float sin = MathUtils.sin(theta);
    float cx = radius * MathUtils.cos(start * MathUtils.degreesToRadians);
    float cy = radius * MathUtils.sin(start * MathUtils.degreesToRadians);

    if (shapeType == ShapeType.Line) {
        check(ShapeType.Line, ShapeType.Filled, segments * 2 + 2);

        renderer.color(colorBits);
        renderer.vertex(x, y, 0);           &lt--- CENTER
        renderer.color(colorBits);
        renderer.vertex(x + cx, y + cy, 0); <--- LINE TO START POINT
        for (int i = 0; i < segments; i++) {
            renderer.color(colorBits);
            renderer.vertex(x + cx, y + cy, 0);
            float temp = cx;
            cx = cos * cx - sin * cy;
            cy = sin * temp + cos * cy;
            renderer.color(colorBits);
            renderer.vertex(x + cx, y + cy, 0);
        }
        renderer.color(colorBits);
        renderer.vertex(x + cx, y + cy, 0); <-- LINE TO END POINT
...

Easiest is probably to copy the entire function and at least throw out two of the lines I marked: CENTER and LINE TO END POINT, along with the accompanying renderer.color(.. line above each.

(You can also delete the LINE TO START POINT –?it appears to set the starting point for the curve, but that's actually also done inside the loop, so it's redundant.)

The function has a second part for a filled "arc" (I agree, it should have properly been named "pie" or "wedge"), but you don't need that here because it would do exactly the same.

If you get it to work, you could propose it to libgdx's maintainers, for example on libgdx's Contributions Forum.