Problem with my RGB to HSV algorithm... casting?

Forum 2005-2010 (read only) Software Syntax & Programs
1

Hey guys,

I am having problems with this common algorithm for converting RGB color values (in the range of 0-255) to HSB (called HSV) values with hue from 0-360 and saturation 0-255, and V 0-255.

The algorithm works except when I have colors where R =255, G=0 and B = anything.

Can anyone see anything I am doing wrong here??

////////////////////////////////////////////////////////////////////////////////
//  Calculates and stores global HSB values when a new RGB color is stored. 
//  This is needed for the fade function to work properly
///////////////////////////////////////////////////////////////////////////////

void calcHSV(){
  
  float var_Min;
  float var_Max;
  float del_Max;
    
  float var_R;
  float var_G;
  float var_B;
  
  float del_R;
  float del_G;
  float del_B;
  
  var_R = ((float)color[R] / 255.0 );                   //RGB from 0 to 255
  var_G = ((float)color[G] / 255.0 );
  var_B = ((float)color[B] / 255.0 );

  var_Min = min( min(var_R, var_G) , var_B );    //Min. value of RGB
  var_Max = max( max(var_R, var_G) , var_B );   //Max. value of RGB
  del_Max = var_Max - var_Min;                  //Delta RGB value

  hsb[V] = (float)var_Max * 255.0;

  if ( del_Max == 0 )                    //This is a gray, no chroma...
  {
     hsb[H] = 0;                                //HSV results from 0 to 1
     hsb[S] = 0;
  }
  else                                    //Chromatic data...
  {
     hsb[S] = ((float)del_Max / (float)var_Max) * 255.0;

     del_R = ( ( ( var_Max - var_R ) / 6.0 ) + ( del_Max / 2.0 ) ) / del_Max;
     del_G = ( ( ( var_Max - var_G ) / 6.0 ) + ( del_Max / 2.0 ) ) / del_Max;
     del_B = ( ( ( var_Max - var_B ) / 6.0 ) + ( del_Max / 2.0 ) ) / del_Max;

     if      ( var_R == var_Max ) hsb[H] = (del_B - del_G) * 360.0;
     else if ( var_G == var_Max ) hsb[H] = (( 1 / 3.0 ) + del_R - del_B) * 360.0;
     else if ( var_B == var_Max ) hsb[H] = (( 2 / 3.0 ) + del_G - del_R) * 360.0;

     if ( hsb[H] < 0 ) ; hsb[H] += 360.0;
     if ( hsb[H] > 1 ) ; hsb[H] -= 360.0;
  }
  
}

This is the code I translated this from:

2

At first glance

if ( hsb[H] < 0 ) ; hsb[H] += 360.0;
if ( hsb[H] > 1 ) ; hsb[H] -= 360.0;

Is probably not what is meant.

if (a > 0) ; b = 7;

Will always assign 7 to b because the first ';', after the ')' terminates an empty statement, which might be puzzling, but is a legal statement.

You probably mean

if ( hsb[H] < 0 ) hsb[H] += 360.0;
if ( hsb[H] > 1 ) hsb[H] -= 360.0;

I almost always use { ... } so I'd write

if ( hsb[H] < 0 ) { hsb[H] += 360.0; }
if ( hsb[H] > 1 ) { hsb[H] -= 360.0; }

Also, the code is incomplete. I can't see what type hsb[], H, S and V.

HTH
GB