Need hint about color conversion

[Wolfgang]

Soquili,

I believe HSB is the same as HSL. But I am using PHP as a programming language and I have no knowledge of VB nor do I have the program.
I did not find a respective function in PHP.

Gary,
about.com works again for me and I found the article by Judy Litt. I just asked her for permission to use her schemes and I am awaiting her response.
My problem with the conversion is, that I need to program a fuction that does the conversion. Just like if you wanted the squareroot of something. You type in your number and the function gives the squareroot back. But I can't use any graphics package to just 'switch' - I need a way to program it myself.

Quaker,
I must admit, I have no clue how to use your sheet, but it seems that I would need something just like that for HSL/HSB <-> RGB to 'peel out' the formula.

By the way, in Xara there is a HSV-mode, but it seems to be different from HSB/HSL. Yet another color model ???

Joroho: You once asket Judy for a permission, did she ever answer ??

Wolfgang

[Soquili]

Hi Wolfgang,

I'm not familiar with PHP, but if it has if-then-else types of statements and can compare Single Precision numeric values then I'll make a plain english description of the steps to take to make the HSL conversion.

It will be later today (after I get home from work) before I can do that.

Soquili

[Wolfgang]

Thanks Soquili,

that sounds like just what I need.

Am I right, that the Triads are easily calculated in HSL/HSB if I just add 120 and 240 to the current Hue ??

Wolfgang

[Soquili]

Wolfgang,

I'll verify the adding 120 and 240 for the triads when I start the translation. If my memory serves me correctly (not usual for me as I get older) then yes that is how triads are calculated. If the result is greater than 360 you would simple subtract 360 to get the correct value.

Soquili

[bjwe]

Wolfgang,

When you add a palette file to the XaraX palette directory it shows up in the Colour Gallery after you re-open XaraX.

The algorithm I used when developing XPal is below, if this helps...

Brian

RGB to HSV & HSV to RGB

The Hue/Saturation/Value model was created by A. R. Smith in 1978. It is based on such intuitive color characteristics as tint, shade and tone (or family, purety and intensity). The coordinate system is cylindrical, and the colors are defined inside a hexcone. The hue value H runs from 0 to 360º. The saturation S is the degree of strength or purity and is from 0 to 1. Purity is how much white is added to the color, so S=1 makes the purest color (no white). Brightness V also ranges from 0 to 1, where 0 is the black.

There is no transformation matrix for RGB/HSV conversion, but the algorithm follows:

// r,g,b values are from 0 to 1
// h = [0,360], s = [0,1], v = [0,1]
//if s == 0, then h = -1 (undefined)

void RGBtoHSV( float r, float g, float b, float *h, float *s, float *v )
{
float min, max, delta;

min = MIN( r, g, b );
max = MAX( r, g, b );
*v = max;// v

delta = max - min;

if( max != 0 )
*s = delta / max;// s
else {
// r = g = b = 0// s = 0, v is undefined
*s = 0;
*h = -1;
return;
}

if( r == max )
*h = ( g - b ) / delta;// between yellow & magenta
else if( g == max )
*h = 2 + ( b - r ) / delta;// between cyan & yellow
else
*h = 4 + ( r - g ) / delta;// between magenta & cyan

*h *= 60;// degrees
if( *h < 0 )
*h += 360;

}

void HSVtoRGB( float *r, float *g, float *b, float h, float s, float v )
{
int i;
float f, p, q, t;

if( s == 0 ) {
// achromatic (grey)
*r = *g = *b = v;
return;
}

h /= 60;// sector 0 to 5
i = floor( h );
f = h - i;// factorial part of h
p = v * ( 1 - s );
q = v * ( 1 - s * f );
t = v * ( 1 - s * ( 1 - f ) );

switch( i ) {
case 0:
*r = v;
*g = t;
*b = p;
break;
case 1:
*r = q;
*g = v;
*b = p;
break;
case 2
*r = p;
*g = v;
*b = t;
break;
case 3:
*r = p;
*g = q;
*b = v;
break;
case 4:
*r = t;
*g = p;
*b = v;
break;
default:// case 5:
*r = v;
*g = p;
*b = q;
break;
}

}

[Wolfgang]

Hi Brian,

that's exactly what I meant, BUT:

I managed to convert the syntax of your example (C ??) to PHP, where it looks like this:

function RGBtoHSV($r,$g,$b)
{
$min = MIN( $r, $g, $b );
$max = MAX( $r, $g, $b );
$v = $max;// v
$delta = $max - $min;

if( $max != 0 )
{
$s = floor(($delta / $max) * 255);// s
}
else
{
$s = 0;
$h = -1;
}

if( $r == $max )
{
@$h = ( $g - $b ) / $delta;// between yellow & magenta
}
else if( $g == $max )
{
@$h = 2 + ( $b - $r ) / $delta;// between cyan & yellow
}
else
{
@$h = 4 + ( $r - $g ) / $delta;// between magenta & cyan
}

$h *= 60;// degrees
if( $h < 0 )
{
$h += 360;
}
$h = round($h,1);

// output for testing
print "H: " . $h . "
";
print "S: " . $s . "
";
print "V: " . $v . "
";
}


Maybe your example has an error or I misunderstand something. But your original

*s = delta / max;// s

gives fractions of 1 as result, while (according to Xara X's color editor) it should be between 0 and 255.
I changed the line like this

$s = floor(($delta / $max) * 255);// s

and now it gives the same result as in X's color editor.

Still I have a slight problem with the "H"-value.
The darker the color is, the more is the difference between Xara's value and mine.

For instance:
<font size='1'>
<table border=1><tr><td>
RGB-color</td><td>
My values (HSV)</td><td>
Xara's values (HSV)</td><tr><tr>
<td>204, 204, 153</td><td>60, 63, 204</td><td>60, 63, 204</td></tr><tr><td>153, 153, 204</td><td>240, 64, 204</td><td>240, 64, 104</td></tr><tr><td>84, 118, 98</td><td>144.7, 74, 118</td><td>144.4, 74, 118</td></tr><tr><td>30, 19, 46</td><td>264.4, 149, 46</td><td>265.7, 149, 46</td></tr></table>
</font>
As you can see, the result differs, the darker the color gets. Do you think, this is a rounding error or something ???

Apart from that, I am quite happy with the solution. I don't need the other way around, RGB->HSV alone is fine.

Still missing: RGB -> HSB/HSL, which is different from HSV ...

Thanks

Wolfgang

P.S.: Don't know why the font inside the table is so big ??

[This message was edited by Wolfgang on February 06, 2002 at 12:09.]

[noticies]

If you thought of a colour-harmony utility, just in case, check out these two websites:

Easy RGB: http://www.easyrgb.com
Color Schemer: http://www.colorschemer.com

Perhaps it isn't necessary to reinvent the wheel ;-)

-------------------------
Joan M. Mas
TypePhases: Dingbats and Fonts for yer computor!
http://inicia.es/de/jmas
-------------------------

[gwpriester]

These are pretty much the same near as I can tell. HSB is Hue Saturation and Brightness, while HSL is Hue Saturation and Lightness. And there is also HSV Hue Saturation and Value.

Brightness and Lightness both refer to the value of the color, or the amount of black to white in the color.

Gary

Gary Priester

Moderator Person

<a href="http://www.gwpriester.com">
www.gwpriester.com </a>


XaraXone

[Soquili]

Hi Wolfgang,

In Brian's code the red, green, and blue need to be converted to percentages before being used in the code. Simply divide the red, green, and blue values by 255 to get their percentages.

HSV and HSL are different colour definitions. As promised, here's the "plain english" algorithm for HSL:

RGB TO HSL

Convert Red, Green, and Blue to percentages

PercentRed = Red divided by 255
PercentGreen = Green divided by 255
PercentBlue = Blue divided by 255

Determine maximum and minimum percentages

SingleMax = Maximum of (PercentRed, PercentGreen, PercentBlue)
SingleMin = Minimum of (PercentRed, PercentGreen, PercentBlue)

Determine the Delta

SingleDelta = SingleMax - SingleMin

Luminosity = (SingleMax + SingleMin) divided by 2

If SingleMax = SingleMin then
Saturation = 0
else
Saturation = 1
end if

If Luminosity <= 0.5 then
If Saturation > 0 then
Saturation = SingleDelta / (SingleMax + SingleMin)
end if
else
If Saturation > 0 then
Saturation = SingleDelta / (2 - SingleMax - SingleMin)
end if
end if

If Saturation != 0 then
If PercentRed = SingleMax then
Hue = (PercentGreen - PercentBlue) / SingleDelta
end if
If PercentGreen = SingleMax then
Hue = 2 + (PercentBlue - PercentRed) / SingleDelta
end if
if PercentBlue = SingleMax then
Hue = 4 + (PercentRed - PercentGreen) / SingleDelta
end if
Hue = Hue * 60
end if

If Hue < 0 then
Hue = Hue + 360
end if


Soquili

P.S. unfortunately the forum removed all the formatting of the text. Hope it's still readable as code.

[michaelward]

Programming and Xara in the same place.

I could be in heaven!



Michael Ward
http://www.metalandplastics.co.uk
http://www.leighcenturions.net