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javaswingopenglawtslick2d

Drawing an arc backwards?

I am porting some of the Slick Graphics class to work with AWT, but I've run into a problem. Slick, being run on OpenGL, draws arcs clockwise. AWT defines them as being drawn counter-clockwise, like in math. I'm wondering how I can draw Slick's arc counter-clockwise, so that it matches AWT.

An image of what I'd expect:

http://i.imgur.com/mmMHjhU.png

An image of what I get:

enter image description here

Solution

  • This is the function to draw an arc in Slick2D:

    public void drawArc(float x1,
                    float y1,
                    float width,
                    float height,
                    float start,
                    float end)

    Since it appears that Slick draws these arcs clock-wise (0 - 360 degrees) then you can do some simple arithmetic to get the effect that you wish.

    enter image description here

    A and B are your original angles that you wish to draw the arc in between like 0 and 90 degrees. drawArc(0,0,1,1,A,B) will render an arc from A to B, but we want an arc from C to D which is the same arc as A to B but counter-clock wise. Since you already know the values of A and B (like 0 and 90) you can get the values for C and D with the following:

    C = 360 - B

    D = 360 - A

    So now we can just use drawArc(0,0,1,1,C,D).

    Test it with values:

    A = 0

    B = 90

    C = 360 - 90 = 270

    D = 360 - 0 = 360

    This will plot an arc from 270 to 360 which is the same as an arc from 0 to 90 counter-clock wise.