On Sep 28, 2010, at 6:32 PM, Florian Höch wrote:
> Hi,
>
> did you notice that the first column does not end at the actual maximum
> (65535)? The cmsBuildGamma values are correct.
>
> for (int i = 0; i < numberOfEntries; i++) {
> ...
> value = (value * max) / (numberOfEntries - 1);
> ...
> }
oh! I should have seen that. Especially if I analyzed the numbers. Take the
last case, for example.
value = 99.0
value = (99.0 * 65535) / 100;
yup, that's not right!
Thanks for the quick response.
Paul
>
> Am 28.09.2010 23:33, schrieb Paul Cezanne:
>> I'm looking at a 1% error in what I expect my gamma to be and now I'm
>> wondering if it is an LCMS bug! Run this program.
>>
>> #include "lcms.h"
>>
>> int main (int argc, const char * argv[]) {
>>
>>
>> const int numberOfEntries = 100;
>>
>> LPGAMMATABLE transferFunctionsDebug;
>> transferFunctionsDebug = cmsBuildGamma(numberOfEntries, 1.0);
>>
>> int max = transferFunctionsDebug[0].GammaTable[numberOfEntries-1];
>>
>> float value;
>> int vInt;
>>
>>
>> for(int i =0; i < 100; i++) {
>> value = i; // convert to float
>> value = (value * max)/numberOfEntries; // normalize to gamma range
>> vInt = (int) (value + 0.5);
>>
>>
>> fprintf(stdout, "%d\t%d\n", vInt, transferFunctionsDebug->GammaTable[i]);
>> }
>>
>> }
>>
>>
>> *
>> I get this output:
>>
>> 0 0
>> 655 662
>> 1311 1324
>> ...
>> 62914 63549
>> 63569 64211
>> 64224 64873
>> 64880 65535
>>
>> I would have expected the columns to be the same. Does cmsBuildGamma not
>> build a perfectly linear LUT?
>
> --
> Florian Höch
>
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