Draw an arc with graphics.library

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Chris

Posted on: 2012/10/22 22:02 #1

Amigans Defender

What's the best way to draw an arc using graphics.library, given two angles in degrees which depict the starting and ending angle of the arc from the horizontal?

I came up with this unoptimised monstrosity, which unsurprisingly doesn't work:


void arc_gfxlib(int x, int y, int radius, int angle1, int angle2, int quarter)

{

    double angle1_r = angle1 * (M_PI / 180.0);

    double angle2_r = angle1 * (M_PI / 180.0);

    double angle, b, c;

    double step = (angle2_r - angle1_r) / ((angle2_r - angle1_r) * (double)radius);

    int x0, y0, x1, y1;



    switch(quarter) {

        case 1:

            x0 = x;

            y0 = y;

        break;

        

        case 2:

            x0 = -x;

            y0 = y;

        break;

        

        case 3:

            x0 = -x;

            y0 = -y;

        break;

        

        case 4:

            x0 = x;

            y0 = -y;

        break;

    }

    

    b = angle1_r;

    c = angle2_r;

    

    for(angle = b; angle <= c; angle += step) {

        x1 = (int)(cos(angle) * (double)radius);

        y1 = (int)(sin(angle) * (double)radius);

        WritePixel(rp, x0 + x1, y0 + y1);

    }

}



void arc(int x, int y, int radius, int angle1, int angle2)

{

    int a1, a2;



    a1 = angle1;

        

    if(angle1 <= 90) {

        if(angle2 > 90) a2 = 90;

            else a2 = angle2;

        arc_gfxlib(x, y, radius, a1, a2, 1);

        a1 = 90;

    }

    

    if(angle1 <= 180) {

        if(angle2 > 180) a2 = 180;

            else a2 = angle2;

        if((angle2 > 90) && angle2 <= 180)

            arc_gfxlib(x, y, radius, a1, a2, 2);

        a1 = 180;

    }

    

    if(angle1 <= 270) {

        if(angle2 > 270) a2 = 270;

            else a2 = angle2;

        if((angle2 > 180) && angle2 <= 270)

            arc_gfxlib(x, y, radius, a1, a2, 3);

        a1 = 270;

    }



    if(angle2 > 270) {

        arc_gfxlib(x, y, radius, a1, angle2, 4);

    }

}

broadblues

Re: Draw an arc with graphics.library

Posted on: 2012/10/22 23:34 #2

Home away from home

Quote:

What's the best way to draw an arc using graphics.library, given two angles in degrees which depict the starting and ending angle of the arc from the horizontal?

I would use a paramtric approach, to determine coordinates on the arc, then draw straight llines between each set of coordinates. Set the ineterval small eneough to get a smooth arc.

Also this looks suspect.

(angle2_r - angle1_r) / ((angle2_r - angle1_r) * (double)radius);

if I'm not reading it wrong don;t the terms in (angle2_r - angle1_r) cancel leaving you with just

1 / (double)radius

ouch and

double angle1_r = angle1 * (M_PI / 180.0);
double angle2_r = angle1 * (M_PI / 180.0);

means that angle2_r = angle1_r

Unless you made a typo whilst posting that's definetly not goining to work!

http://www.mathopenref.com/coordparamellipse.html
http://www.mathopenref.com/coordcirclealgorithm.html

Blender For OS4.x : Blues : Walker Broad

Chris

Re: Draw an arc with graphics.library

Posted on: 2012/10/22 23:49 #3

Amigans Defender

Quote:

broadblues wrote:

double angle1_r = angle1 * (M_PI / 180.0);
double angle2_r = angle1 * (M_PI / 180.0);

means that angle2_r = angle1_r

Unless you made a typo whilst posting that's definetly not goining to work!

Oops. That might explain why I was only getting a dot rather than an arc.

I'll fix that tomorrow and see if I get more expected results.

billyfish

Re: Draw an arc with graphics.library

Posted on: 2012/10/23 12:42 #4

Just popping in

Hi Chris

The algorithm I used is based upon Bresenham's (http://en.wikipedia.org/wiki/Bresenham's_line_algorithm) and is at http://en.wikipedia.org/wiki/Midpoint_circle_algorithm. If you're looking for a smooth transition between 2 points then quaternions work well as they allow you to smoothly interpolate motion in a way that matrices can't

cheers

billy

Chris

Re: Draw an arc with graphics.library

Posted on: 2012/10/23 20:08 #5

Amigans Defender

Quote:

Chris wrote:
I'll fix that tomorrow and see if I get more expected results.

Yes, it works now, with some tweaks (such as remembering screen y coordinates are upside down) - thanks!

Quote:

billyfish wrote:
The algorithm I used is based upon Bresenham's

I think that was what I looked at originally. The examples always draw a whole circle, and how to get it to only plot an arc is a bit vague. It's likely miles faster than using trigonometry for each point though, so it'll be good to get that working.

billyfish

Re: Draw an arc with graphics.library

Posted on: 2012/10/23 20:35 #6

Just popping in

Quote:

Chris wrote:

Quote:
billyfish wrote:
The algorithm I used is based upon Bresenham's

I think that was what I looked at originally. The examples always draw a whole circle, and how to get it to only plot an arc is a bit vague. It's likely miles faster than using trigonometry for each point though, so it'll be good to get that working.

It works for arc sections too, take a look at the "Drawing incomplete octants" on the midpoint circle algorithm page.

If you can get a copy of Computer Graphics by Foley et al. it really is a goldmine of useful stuff (http://www.amazon.co.uk/Computer-Grap ... TF8&qid=1351020762&sr=1-3)

cheers

billy

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