Gets the image histogram

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- PHP Manual
- Imagick
- Gets the image histogram

# Imagick::getImageHistogram

(PECL imagick 2, PECL imagick 3)

Imagick::getImageHistogram — Gets the image histogram

### Description

public Imagick::getImageHistogram(): array

Returns the image histogram as an array of ImagickPixel objects.

### Parameters

This function has no parameters.

### Return Values

Returns the image histogram as an array of ImagickPixel objects.

### Errors/Exceptions

Throws ImagickException on error.

### Examples

Example #1 Generates Imagick::getImageHistogram()

    <?php
    function getColorStatistics($histogramElements, $colorChannel) {
        $colorStatistics = [];

        foreach ($histogramElements as $histogramElement) {
            $color = $histogramElement->getColorValue($colorChannel);
            $color = intval($color * 255);
            $count = $histogramElement->getColorCount();

            if (array_key_exists($color, $colorStatistics)) {
                $colorStatistics[$color] += $count;
            }
            else {
                $colorStatistics[$color] = $count;
            }
        }

        ksort($colorStatistics);

        return $colorStatistics;
    }

    function getImageHistogram($imagePath) {

        $backgroundColor = 'black';

        $draw = new \ImagickDraw();
        $draw->setStrokeWidth(0); //make the lines be as thin as possible

        $imagick = new \Imagick();
        $imagick->newImage(500, 500, $backgroundColor);
        $imagick->setImageFormat("png");
        $imagick->drawImage($draw);

        $histogramWidth = 256;
        $histogramHeight = 100; // the height for each RGB segment

        $imagick = new \Imagick(realpath($imagePath));
        //Resize the image to be small, otherwise PHP tends to run out of memory
        //This might lead to bad results for images that are pathologically 'pixelly'
        $imagick->adaptiveResizeImage(200, 200, true);
        $histogramElements = $imagick->getImageHistogram();

        $histogram = new \Imagick();
        $histogram->newpseudoimage($histogramWidth, $histogramHeight * 3, 'xc:black');
        $histogram->setImageFormat('png');

        $getMax = function ($carry, $item)  {
            if ($item > $carry) {
                return $item;
            }
            return $carry;
        };

        $colorValues = [
            'red' => getColorStatistics($histogramElements, \Imagick::COLOR_RED),
            'lime' => getColorStatistics($histogramElements, \Imagick::COLOR_GREEN),
            'blue' => getColorStatistics($histogramElements, \Imagick::COLOR_BLUE),
        ];

        $max = array_reduce($colorValues['red'] , $getMax, 0);
        $max = array_reduce($colorValues['lime'] , $getMax, $max);
        $max = array_reduce($colorValues['blue'] , $getMax, $max);

        $scale =  $histogramHeight / $max;

        $count = 0;
        foreach ($colorValues as $color => $values) {
            $draw->setstrokecolor($color);

            $offset = ($count + 1) * $histogramHeight;

            foreach ($values as $index => $value) {
                $draw->line($index, $offset, $index, $offset - ($value * $scale));
            }
            $count++;
        }

        $histogram->drawImage($draw);

        header( "Content-Type: image/png" );
        echo $histogram;
    }

    ?>