How do you convert an image to black and white in PHP

Question

How does one go about converting an image to black and white in PHP?

Not just turning it into greyscale but every pixel made black or white?

Answer 1

For each pixel you must convert from color to greyscale - something like $grey = $red * 0.299 + $green * 0.587 + $blue * 0.114; (these are NTSC weighting factors; other similar weightings exist. This mimics the eye's varying responsiveness to different colors).

Then you need to decide on a cut-off value - generally half the maximum pixel value, but depending on the image you may prefer a higher value (make the image darker) or lower (make the image brighter).

Just comparing each pixel to the cut-off loses a lot of detail - ie large dark areas go completely black - so to retain more information, you can dither. Basically, start at the top left of the image: for each pixel add the error (the difference between the original value and final assigned value) for the pixels to the left and above before comparing to the cut-off value.

Be aware that doing this in PHP will be very slow - you would be much further ahead to find a library which provides this.