Legacy ImageMagick Discussions Archive

1. Including a percentage sign anywhere in the color forces the parser to assume all values are percentages without informing the user. So the user thinks they are getting rgb(240,15,50%), but really they are getting rgb(240%,15%,50%).
2. Most definitions of the HSL and HSV color spaces specifically require coordinate asymmetry. The original paper* identifies that "hue is a circular or modular quality, while all the other characteristics mentioned imply the existence of minimum and maximum values" (and hence might be interpreted as scales or percentages). They ultimately place the saturation and lightness components on a scale from 0 to 1 and hue as an angle, but note that the "scaling ... is somewhat arbitrary".
   
   While IM implements an asymmetric numeric format (hue/360, sat/256, light/256), due to #1, entering any of the coordinates as a percentage will shift the entire format to a symmetrical one (hue/100%, sat/100%, light/100%). Thus a naive user following a traditional HSL specification might get unexpected colors, because it would be natural to enter S&L or S&V as percentages of their maximum value, but leave hue as a degree (or radian) value, since it does not have a clear "maximum".
   
   The CSS3 specification (http://dev.w3.org/csswg/css-color/#the-hsl-notation) is particularly troublesome because it only accepts the format the format "hsl(<hue>,<percentage>,<percentage>)". I don't know for certain, but I suspect CSS is one of the primary ways people are exposed to HSL.

• Joblove, George H., and Donald Greenberg. "Color spaces for computer graphics." ACM siggraph computer graphics. Vol. 12. No. 3. ACM, 1978. (http://www.ann.jussieu.fr/frey/papers/s ... aphics.pdf)

fmw42 wrote:As far as I know, this is currently the way IM works. You have a choice of either 0-360 for hue and 0-255 for sat and lightness/brightness/intensity. Or all 3 are in the range of 0 to 100%. Think of the latter as range 0 to 1 for all 3 but multiplied by 100.

fmw42 wrote:I would suspect that this percent parsing goes deep in the guts of IM and may not be easily changed.

flags=NoValue;
[...]
for (p=pedantic_geometry; *p != '\0'; )
  {
    [...]
    switch (c)
    {
      case '%':
      {
        flags|=PercentValue;
        (void) CopyMagickString(p,p+1,MaxTextExtent);
        break;
      }
[...]
    }
  }

          if ((LocaleCompare(colorspace,"HCL") == 0) ||
              (LocaleCompare(colorspace,"HSB") == 0) ||
              (LocaleCompare(colorspace,"HSL") == 0) ||
              (LocaleCompare(colorspace,"HWB") == 0))
            {
              PixelPacket
                pixel;

              scale=1.0/360.0;
              if ((flags & PercentValue) != 0)
                scale=1.0/100.0;
              geometry_info.rho*=360.0*scale;
              scale=1.0/255.0;
              if ((flags & PercentValue) != 0)
                scale=1.0/100.0;
              geometry_info.sigma*=scale;
              geometry_info.xi*=scale;
              [...]
            }

zweibieren wrote:THIS IS NOT an enhancement request. Imagemagick got broken at some point. My old script used hsl(degree, number, number). It worked fine. When I try to use it now, IT FAILS. See the documentation http://www.imagemagick.org/MagickStudio/Color.html.