Section 16.14. Special Effects Using imagefilter( )

16.14. Special Effects Using imagefilter( )

The filters described here were written for the PHP-bundled build of GD, and may not be available in other releases.

The best way to explain this function is to describe how it works, then show a code example. Although the function accepts different numbers of parameters that do very different things, the function returns TRue if the filter was applied successfully and false otherwise.

First up is IMG_FILTER_BRIGHTNESS, which takes a number between -255 and 255 that represents how much you want to brighten or darken the image. Setting it to 0 leaves the picture unchanged, 255 sets it to full white (brightest), and -255 sets it to full black (darkest). Most pictures tend to look almost invisible beyond +200 or -200.

This code example will lighten our space picture just a little:

     $image = imagecreatefrompng("space.png");     imagefilter($image, IMG_FILTER_BRIGHTNESS, 50);     header("content-type: image/png");     imagepng($image);     imagedestroy($image); 

Next up is IMG_FILTER_COLORIZE, which takes three parameters between -255 and 255 that respectively represent the red, green, and blue values you want to add or subtract from the image. Setting the blue value to -255 will take all the blue out of all the pixels in the image, whereas setting the red to 128 will add red to them. Setting all three of them to 128 will have the effect of adding white to the picture, brightening it in the same way as IMG_FILTER_BRIGHTNESS.

This code example will make our image look more magenta:

     $image = imagecreatefrompng("space.png");     imagefilter($image, IMG_FILTER_COLORIZE, 100, 0, 100);     header("content-type: image/png");     imagepng($image);     imagedestroy($image); 

Moving on, the IMG_FILTER_CONTRAST filter allows you to change the contrast of the image, and takes just one parameter for a contrast value between -255 and 255. Lower values increase the contrast of the picture, essentially reducing the number of colors so that they are more separate and obvious to the eye. Using positive values brings the colors closer together by mixing them with gray until, at 255, you have a full-gray picture.

This code example shows how even a small positive number makes quite a difference to the resulting image:

     $image = imagecreatefrompng("space.png");     imagefilter($image, IMG_FILTER_CONTRAST, 20);     header("content-type: image/png");     imagepng($image);     imagedestroy($image); 

The IMG_FILTER_EDGEDETECT and IMG_FILTER_EMBOSS filters make all the edges in your picture stand out as if they were embossed, and sets everything else to gray. No parameters are needed for either of them, so using them is quite easy.

This next script uses edge detection to grab the edges, then embosses them to make the effect more obvious:

     $image = imagecreatefrompng("space.png");     imagefilter($image, IMG_FILTER_EDGEDETECT);     imagefilter($image, IMG_FILTER_EMBOSS);     header("content-type: image/png");     imagepng($image);     imagedestroy($image); 

If you want to blur an image, you have a choice of two filters: IMG_FILTER_GAUSSIAN_BLUR and IMG_FILTER_SELECTIVE_BLUR. The latter is a generic blur function, and the former is a classic "out-of-focus lens" technique that often actually enhances images. Neither function requires parameters.

Although they're easy to use, there's no harm showing an examplehere are both of them in action. Just comment out the one you don't want to see:

     $image = imagecreatefrompng("space.png");     imagefilter($image, IMG_FILTER_GAUSSIAN_BLUR);     imagefilter($image, IMG_FILTER_SELECTIVE_BLUR);     header("content-type: image/png");     imagepng($image);     imagedestroy($image); 

There's a similar filter, IMG_FILTER_SMOOTH, which gives you a little more control over the output. It takes one parameter, but it takes a little explanation! Unlike the other parameters so far, this isn't a value pertaining to how much you'd like to smooth the image. Instead, it's a weighting for an image manipulation matrix, and small changes can affect the output massively.

There isn't enough room here to go into a full discussion of what these manipulation matrices are, but suffice to say you can represent many different transformationsfrom Gaussian blur to edge detectionusing a 3 x 3 numerical matrix, that defines how the colors of the eight pixels surrounding any given pixel (with the pixel itself being the ninth) should have their RGB values changed. With IMG_FILTER_SMOOTH, the parameter you pass is used as the change value for the pixel itself, which means you get to define how much the pixel's own color is used to form its final color.

You're not likely to want values outside of the range -8 to 8, as even one number makes quite a big difference. At about 10, the picture is almost normal, because the original pixel values are given more weight than the combined sum of its neighbors. But you can get some cool effects between -6 to -8.

This code example smooths the picture just a little:

     $image = imagecreatefrompng("space.png");     imagefilter($image, IMG_FILTER_SMOOTH, 6);     header("content-type: image/png");     imagepng($image);     imagedestroy($image); 

There are two helpful filters that alter the colors in a simple way, which are IMG_FILTER_GRAYSCALE and IMG_FILTER_NEGATE. Both take no parameters: the first sets the picture to grayscale, and the second sets it to use negative colors.

This code example changes the picture to grayscale, then flips it to negative colors:

     $image = imagecreatefrompng("space.png");     imagefilter($image, IMG_FILTER_GRAYSCALE);     imagefilter($image, IMG_FILTER_NEGATE);     header("content-type: image/png");     imagepng($image);     imagedestroy($image);