Opaque using fill for the replacement colour and -fuzz to select a range.

$cmd = "$input -fuzz \"20%\" -fill orange -opaque red"; 
exec("convert $cmd opaque.jpg");

Dither the image using a pre-defined ordered dither threshold map specified, and a uniform color map with the number of levels per color channel.

Specify orientation of a digital camera image.

$cmd = "$input -orient bottom-left"; 
exec("convert $cmd orient.jpg");
 

Set the size and location of an image on the larger virtual canvas.

Set the size and location of an image on the larger virtual canvas.

Oil paint effect.

$cmd = "$input -paint 2"; 
exec("convert $cmd paint.jpg");

Write images to this path on the disk.

Pause between animation loops.

set each pixel whose value is less than |epsilon| to -epsilon or epsilon (whichever is closer) otherwise the pixel value remains unchanged.

Efficiently determine image characteristics.

$output = exec("identify -ping -format \"%[fx:h*0.2]\" $input"); 
echo $output;
 

( This can speed up the identify process. )

Set the font size to be used with draw, annotate etc.

$cmd = "$input -font font/DRAGM.ttf -pointsize 55 ". 
"-fill white-gravity south -annotate 0,0 \"Draw text\" "; 
exec("convert $cmd pointsize.jpg");
 

Polaroid effect.

$cmd = " -caption \"A  Hibiscus\" $input -background black -polaroid 5"; 
exec("convert $cmd polaroid.png");
 

( -polaroid = set the angle +polaroid = random angle between -15 and +15 degrees. - Background controls the shadow colour; save as png for transparent background. )

Combines multiple images according to a weighted sum of polynomials; one floating point weight (coefficient) and one floating point polynomial exponent (power) for each image expressed as comma separa

Reduce the image to a limited number of color levels.

$cmd = "$input -posterize 3"; 
exec("convert $cmd posterize.jpg");