[Haskell-cafe] Line drawing algorithm
Neil Brown
nccb2 at kent.ac.uk
Fri Jul 17 08:50:30 EDT 2009
CK Kashyap wrote:
> Hi All,
>
> I am working on a diagraming utility in Haskell. I started with line
> drawing.
> I am doing the basic stuff using the y = mx + c formula to draw a line
> between (x1,y1) and (x2,y2)
Hi,
Are you doing this to learn Haskell, learn about drawing lines, or to
just get it implemented? If either of the latter two, when drawing a
straight line you shouldn't need to do floating point operations such as
this:
> newY = y1 + round (slope * (fromIntegral (newX - x1)))
Bresenham's algorithm (or similar variants) allows you to draw a line
without needing floating point. A Haskell implementation is here:
http://rosettacode.org/wiki/Bresenham%27s_line_algorithm#Haskell
Although it may not be too understandable! Wikipedia has an explanation
of the general algorithm:
http://en.wikipedia.org/wiki/Bresenham%27s_line_algorithm
As to how to cope with the dy > dx case in your code given the dx > dy
case, you could just swap the x and y coords at the start, then swap
back the x and y coords of all the output points afterwards. Odd, but
effective :-)
Thanks,
Neil.
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