Bob Klimek klimek at grc.nasa.gov
Mon Apr 11 21:27:52 CEST 2005

```Ray Pasco wrote:

> I've been playing around with just this. This program converts to/from
> several different colorspaces and changes the alternate colorspace's
> "ramp" slope and offset. The program only does linear adjustments.
>
> I'd appreciate any comments/suggestions you may have.

Hi Ray,

I've taken a quick look at your rgb-hsl (and hsl-rgb) and rgb-hsv (and
hsv-rgb) transformations and from the few tests I've run, both
algorithms seem to be working quite well. And both successfully convert
to and from. I don't see any difference really, although I don't have
the time to do a thorough comparison.

My interest is only in hue so I've looked at that component only. I have
a test image of a type of "rainbow" in which the hue changes from 1.0
down to 0.0 in a linear fashion as the image is scanned from left to
right. I use an old algorithm which produces a linear relationship
between hue and posistion in this image. To my surprise both of these
algorithms also produce a linear relationship.

BTW, the algorithm that I use (see below) come from Computer Graphics
Quarterly Report SIGGRAPH-ACM, vol 13, Number 3, Aug. 1979. The hue
range is from 0 to 2pi. One interesting thing we've noticed is that some
hue values repeat them selves. That is an 8-bit deep RGB image has 16
million rgb combinations but our hue algorithm produces only a few
thousand unique values, not 16 million. All others are repeated hue values.

Anyway, it would be very nice to have a fast image-wide HSI
transformation done in C with the output as a float array. Maybe some day...

def calcHue(self, r, g, b):
rf = float(r)
gf = float(g)
bf = float(b)
rf = rf/255.0
gf = gf/255.0
bf = bf/255.0
inten = (rf+gf+bf)/3.0
if(not((r==b) and (r==g))):
min = rf
if(min>gf): min = gf
if(min>bf): min = bf
sat=1.0-(min/inten)
t1 = rf-gf
t2 = rf-bf
t3 = gf-bf
t4 = math.sqrt((t1*t1)+(t2*t3))
t5 = (t1+t2)/2.0
t5 = t5/t4
hue = math.acos(t5)
if(gf<bf): hue=2.0*math.pi-hue
else:
hue=-1000.0
sat=-1.0
return (hue, sat)

Bob

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