[Image-SIG] HSL adjustment, and others

[Bob Klimek]

No. The paper mentioned below has the pseudocode but I've never 
implemented it.
Bob


Ray Pasco wrote:

> Bob,
>
> So, that's HSI ?  OK, but it seems rather bizarre.  Anyhow, do you 
> have the inverse transform ?
>
> Ray
>
>
> Bob Klimek wrote:
>
>> 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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