[Interest] QColor methods saturation(), value() and getHsv() are incredibly slow

[Jochen Pernsteiner]

Hello,

I'm playing around with an application were I use a QImage with a RGB32 format.

I used the QColor method saturation() and value() to compute these values (I do not

need hue), and found out that these functions (and naturally also getHSV()) are incredibly slow.

I wrote a straightforward implementation of a RGB-to-SV conversion, which on my

PC is 12-15 times faster than using QColor's getHSV() method.

Wikipedia has a good explanation of HSV: http://en.wikipedia.org/wiki/HSL_and_HSV

The only difference between my function and the QColor method(s) is that

the LSB of 'saturation' is not always the same. I guess this is a rounding issue.

So my question is:

Are these QColor methods so slow or am I doing something wrong?

(The Qt version I'm using is 4.8.2 (mingw open source) on Windows XP.)

My function looks like this:

typedef unsigned char u8;
typedef unsigned short u16;
typedef unsigned long u32;

//--------------------------------------------------------------

u8 max3(u8 a, u8 b, u8 c)
{
  u8 max = (a > b) ? a : b;
  max = (max > c) ? max : c;
  return max;
}
//--------------------------------------------------------------
u8 min3(u8 a, u8 b, u8 c)
{
  u8 min = (a > b) ? b : a;
  min = (min > c) ? c : min;
  return min;
}
//--------------------------------------------------------------
u16 rgb_to_sv(u32 rgb)
{
  u8 red = (rgb >> 16) & 0xff;
  u8 green = (rgb >> 8) & 0xff;
  u8 blue = rgb & 0xff;

  u8 max = max3(red,green,blue);
  u8 min = min3(red,green,blue);
 
  if(max == 0) return 0;

  double saturation = (double)(max - min)/max;
  double tmp = saturation * 255;
  tmp += 0.5;
  u8 saturation_u8 = tmp;
 
  return (saturation_u8 << 8) + max; // value = max
}

//--------------------------------------------------------------

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