How to convert image to GIF with only two color index in Imagick - imagick

Question

I want to convert an image to GIF format with only two color, black and white
so every pixel in output image will be black or white
does anybody know any way to convert in imagick ?

Answer 1

Although the final conversion to write out a two-color gif are the same, there are subtly different ways of converting an image to be two color black and white.
Here are some methods below:
Helper method to actually force images to be pixels of values 0,0,0 and 255,255,255
function forceBlackAndWhite(Imagick $imagick, $ditherMethod = \Imagick::DITHERMETHOD_NO)
{
$palette = new Imagick();
$palette->newPseudoImage(1, 2, 'gradient:black-white');
$palette->setImageFormat('png');
//$palette->writeImage('palette.png');
// Make the image use these palette colors
$imagick->remapImage($palette, $ditherMethod);
$imagick->setImageDepth(1);
}
Just use the remap to palette to force the image to 2 colors without any dithering.
function twoColorPaletteOnly()
{
$imagick = new Imagick(__DIR__."/../images/Biter_500.jpg");
forceBlackAndWhite($imagick, \Imagick::DITHERMETHOD_NO);
$imagick->setImageFormat('gif');
$imagick->writeImage("./outputPalette.gif");
}
Palette output:
Using the http://phpimagick.com/Imagick/posterizeImage allows different control over the dithering process.
function twoColorViaPosterize()
{
$imagick = new Imagick(__DIR__."/../images/Biter_500.jpg");
$imagick->transformImageColorspace(\Imagick::COLORSPACE_GRAY);
$imagick->posterizeImage(2, \Imagick::DITHERMETHOD_RIEMERSMA);
forceBlackAndWhite($imagick);
$imagick->setImageFormat('gif');
$imagick->writeImage("./outputPosterize.gif");
}
Posterize output:
The thresholdImage function allows us to control at what at 'level' the image changes from black to white.
function twoColorViaThreshold()
{
$imagick = new Imagick(__DIR__."/../images/Biter_500.jpg");
$imagick->transformImageColorspace(\Imagick::COLORSPACE_GRAY);
$imagick->thresholdImage(0.5 * \Imagick::getQuantum());
forceBlackAndWhite($imagick);
$imagick->setImageFormat('gif');
$imagick->writeImage("./outputThreshold.gif");
}
Threshold output:
Using the blackThresholdImage and whiteThresholdImage functions allows us to control the color threshold per channel
function twoColorViaColorThreshold()
{
$imagick = new Imagick(__DIR__."/../images/Biter_500.jpg");
$thresholdColor = "RGB(127, 100, 100)";
$imagick->blackThresholdImage($thresholdColor);
$imagick->whiteThresholdImage($thresholdColor);
forceBlackAndWhite($imagick);
$imagick->setImageFormat('gif');
$imagick->writeImage("./outputColorThreshold.gif");
}
colorThreshold output
Extracting a single image channel can produce a 'cleaner' looking output image.
function twoColorViaColorChannelThreshold()
{
$imagick = new Imagick(__DIR__."/../images/Biter_500.jpg");
$imagick->separateImageChannel(\Imagick::CHANNEL_RED);
$imagick->thresholdImage(0.5 * \Imagick::getQuantum());
forceBlackAndWhite($imagick);
$imagick->setImageFormat('gif');
$imagick->writeImage("./outputColorChannelThreshold.gif");
}
colorChannelThreshold
We can combine the RGB channels more precisely using the colorMatrixImage function, which gives us complete control over how the separate R G B values should affect the output image.
function twoColorViaColorMatrixChannelThreshold()
{
$imagick = new Imagick(__DIR__."/../images/Biter_500.jpg");
// The only
$colorMatrix = [
0.6, 0.2, 0.2, 0.0, 0.0,
0.0, 0.0, 0.0, 0.0, 0.0,
0.0, 0.0, 0.0, 0.0, 0.0,
0.0, 0.0, 0.0, 1.0, 0.0,
0.0, 0.0, 0.0, 0.0, 0.0,
];
// The intensity of the red channel after applying this color matrix takes
// the values from the pixels before the transformation of:
// 60% of the red, 20% blue, 20% green
$imagick->colorMatrixImage($colorMatrix);
$imagick->separateImageChannel(\Imagick::CHANNEL_RED);
$imagick->thresholdImage(0.5 * \Imagick::getQuantum());
forceBlackAndWhite($imagick);
$imagick->setImageFormat('gif');
$imagick->writeImage("./outputColorMatrixChannelThreshold.gif");
}
colorMatrixChannelThreshold output
The output images for the code above are: