Review Request: KColorSpace::KHCY::KHCY(const & QColor) constructor floating point precision error

[Johannes Sixt]

Matthew Woehlke schrieb:
> Benoit Jacob wrote:
>> we seem to be talking of 2 different things. I
>> was talking about the specific function in that specific file, where
>> the color is read off the RGB components of a QColor. Let's paste the
>> code:
>>
>> KHCY::KHCY(const QColor& color)
>> {
>>     qreal r = gamma(color.redF());
>>     qreal g = gamma(color.greenF());
>>     qreal b = gamma(color.blueF());
>>     a = color.alphaF();
>>
>>     // luma component
>>     y = lumag(r, g, b);
>>
>>     // hue component
>>     qreal p = qMax(qMax(r, g), b);
>>     qreal n = qMin(qMin(r, g), b);
>>     qreal d = 6.0 * (p - n);
>>     if (n == p)
>>         h = 0.0;
>>     else if (r == p)
>>         h = ((g - b) / d);
>>     else if (g == p)
>>         h = ((b - r) / d) + (1.0 / 3.0);
>>     else
>>         h = ((r - g) / d) + (2.0 / 3.0);

gamma() is a strictly monotonic function. Therefore, you can look at the
integer RGB values of QColor in order to decide which branch to take. You
don't have to look at the gamma-corrected floating-point RGB values.

>>
>>     // chroma component
>>     if (r == g && g == b)
>>         c = 0.0;
>>     else
>>         c = qMax( (y - n) / y, (p - y) / (1 - y) );
>> }

Same here.

-- Hannes