Terje Mathisen <spamtrap@[EMAIL PROTECTED]
> writes:
> rep_movsd wrote:
>> On May 8, 7:00 pm, Terje Mathisen <spamt...@[EMAIL PROTECTED]
> wrote:
>>> How does it handle the corner/edge cases?
>>
>> The edges are handled by ignoring all pixels which do not have the
>> neighbours, the simplest way I found was to keep my x and y loops
>> ranging over [1, width-1) and [0, height-1).
>
> OK. Quick & dirty indeed.
>>
>>> It seems to me like the getNearestPalColor() function could easily
>>> dominate the processing time here!
>>
>> Well Initially the code used the standard distance matching algorithm
>> as seen in zillions of color reduction libraries :
>>
>> For each color in the palette
>> Compute the RGB colorspace distance between the palette entry and the
>> target color -> (r - rp)^2 + (g - gp)^2 + (b - bp)^2
>> Choose the index with the minimum distance.
>>
>> This was pathetically slow... It has to loop 256 times for each pixel
>> doing several arithmetic ops!
>> One optimization is to use abs and get rid of the multiplications.
>
> But that messes up the distance calculation because it gives the wrong
> answer for many edge cases. OTOH, the error diffusion would tend to
> distribute this error across the neighboring pixels, making the visual
> impact much less.
It doesn't "mess up the distance calculations" at all, it
simply selects a different metric, namely the L_1 metric
rather than the L_2 metric. However, both are "wrong" in
the strict sense, as a metric which models human vision with
any attempt at fidelity would weight the three different
chroma components differently, giving more weight to the
green component, and less to the red component. (Or even
take place in a different colour-space entirely.) The
volume of space for which the unweighted L_2 metric choses
the wrong palate colour (not the nearest according to the
model of human visual perception) will most probably be
larger than the volume of the corner cases where L_2 selects
the correct colour but L_1 doesn't. (And of course,
there will even be times where the L_1 selects a better
palate colour than L_2.)
Phil
--
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