Special FX, Duotone

Once you have mastered the greyscale effect, you are 95% of the way there with duotone. This effect is essentially a tinted greyscale, which means that we average the red, green, and blue values then add or subtract a little from each of them to get a particular tint. In order to handle the tint values, our function needs to accept four parameters: the image, a red amount, green amount, and blue amount. There is one additional check to be made for each colour, and that is that it is not over 255 - the maximum value for a single colour. Here is the new function:

function duotone (&$image, $rplus, $gplus, $bplus) {

      $imagex = imagesx($image);

      $imagey = imagesy($image);

  

      for ($x = 0; $x <$imagex; ++$x) {

          for ($y = 0; $y <$imagey; ++$y) {

              $rgb = imagecolorat($image, $x, $y);

              $red = ($rgb >> 16) & 0xFF;

              $green = ($rgb >> 8) & 0xFF;

              $blue = $rgb & 0xFF;

              $red = (int)(($red+$green+$blue)/3);

              $green = $red + $gplus;

              $blue = $red + $bplus;

              $red += $rplus;

  

              if ($red > 255) $red = 255;

              if ($green > 255) $green = 255;

              if ($blue > 255) $blue = 255;

              if ($red < 0) $red = 0;

              if ($green < 0) $green = 0;

              if ($blue < 0) $blue = 0;

  

              $newcol = imagecolorallocate ($image, $red,$green,$blue);

              imagesetpixel ($image, $x, $y, $newcol);

          }

      }

  }

The first half of that is basically identical to our greyscale function, but note that the new function takes $rplus, $gplus, and $bplus to handle users passing in tinting information. Now, when the colours are average, the value is stored in $red. The $green variable is then set to the value of $red plus the $gplus, which is the green tint amount passed into the function. The same is done for blue, with $blue and $bplus.

Finally, the $red variable itself is changed to reflect the red tint amount, then a new colour is allocated based upon these three and the pixel is coloured in with imagesetpixel().

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