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

You could shell out to imagemagick, assuming your host supports it. What function do you want to use for deciding if a pixel should be black or white?

Answer 2

If you intend to do this yourself, you will need to implement a dithering algorithm. But as @jonni says, using an existing tool would be much easier?

Answer 3

Simply round the grayscale color to either black or white.

float gray = (r + g + b) / 3
if(gray > 0x7F) return 0xFF;
return 0x00;

Answer 4

Using the php gd library:

imagefilter($im, IMG_FILTER_GRAYSCALE);
imagefilter($im, IMG_FILTER_CONTRAST, -100);

Check the user comments in the link above for more examples.

Answer 5

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.