off by 1 error?

[Terry Reedy]

On 4/9/2010 4:10 PM, david jensen wrote:
> Hi all,
>
> I'm trying to assign a color to various values between some arbitrary
> maximum and minimum, and i'd like to go from red to blue to green .
> Currently, I have the following, where i'm passing the minimum and
> maximum as a 2-tuple:
>
> def getcolor( minmax, curr ):

One would need more info re minmax and curr and their relationship to 
definitively answer your question.

> 	rangesize = (minmax[1] - minmax[0]) - 1

I would expect that either +0 or +1 woult be correct

> 	currsize = float( curr - minmax[0] )
> 	currpos = currsize / rangesize
> 	colornum = int( currpos*511 + 0.5 )

The .5 may be the reason for the '1/2' question.

> 	r = max( 255 - colornum, 0 )
> 	g = max( -256 + colornum, 0 )

r,g are both 0 for both 255 and 256. I suspect this is reason for the 
'double' question.

> 	b = 255 - ( r+g )
> 	return r,g,b
>
> 1) is there a better way to do this?
> 2) at the extreme points, this doesn't work: the number of (255, 0,
> 0)s and (0, 255, 0)s are 1/2 what they should be, and (0, 0, 255) is
> double what it should be.
>
> the above algorithm at least doesn't give me things like (0, -1, 256),
> which i struggled with for a while, but where is/are my off-by-one(s)?

Again, start with a complete definition of the params.

Terry Jan Reedy