Drawing circles

[h0ffman]

Just a theoretical thread here. I've been thinking about how people draw circles in Assembly. The theory i thought was...

1. Plot the dots either side with the CPU
2. Blitter fill

I've seen loads of demos where the circles change in size, my guess here is you have a large high definition sine list and pre-calc the points for each different size of circle?

Just wondered if I was on the right lines here....

[pmc]

h0ff - drop me an email so I remember but I'm sure I've got some code for drawing circles that you can either plug into what you're working on or at least use as a springboard for your own routine.

[WayneK]

It depends what you're doing with the circles - what you suggested is one common way to do it (precalc for each radius size, plot+fill) if the circles were smallish - afaik most large circles used the pyramid/scanline method (bitplane contains a large pyramid, until the base is as wide as the widest circle radius you need - then you 'just' show the correct line of the pyramid for each scanline of your circle, again from a precalced list of which widths to show...all those demos with the enormous 1-circle-per-bitplane 'vectorballs' were done this way I believe).

[Thorham]

Originally Posted by h0ffman

I've seen loads of demos where the circles change in size, my guess here is you have a large high definition sine list and pre-calc the points for each different size of circle?

No need for sine lists! Check out the mid-point algorithm: http://en.wikipedia.org/wiki/Midpoint_circle_algorithm. Precalculating the circles sounds like a fine plan, and you only need a quarter of each circle.

[h0ffman]

Cool, was just checking the Assembly theory I had in my head was what right, well, in relation to amiga assembly and hardware at least

I like the bitplane / pyramid idea, you could probably have some super phat AGA circle drawn with that !