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().

Copyright ©2015 Paul Hudson. Follow me: @twostraws.