How to change Image to jpg and set dpi to 300 using php?

Question

I have following code in a controller under pngtojpgAction() which I am calling using ajax.

Through this

$this->getRequest()->getParam('imagedata'));

statement I am getting a pattern like this data:image/jpeg;base64,/9j/4AAQSkZJR......AH9T796KtUV1HGf/Z

Which is a png image data.

now I am using following code to convert this png image to jpeg and to increase dpi to 300.

public function pngtojpgAction()   
{
    //Code to convert png to jpg image
    $input = imagecreatefrompng($this->getRequest()->getParam('imagedata'));
    $width=imagesx($input);
    $height=imagesy($input);
    $output = imagecreatetruecolor($width, $height);
    $white = imagecolorallocate($output,  255, 255, 255);
    imagefilledrectangle($output, 0, 0, $width, $height, $white);
    imagecopy($output, $input, 0, 0, 0, 0, $width, $height);  
     
    ob_start();
    imagejpeg($output);
    $contents =  ob_get_contents();
    ob_end_clean();
     //echo 'data:image/jpeg;base64,'.base64_encode($contents); /*Up to here code works well*/
    
     $jpgImage='data:image/jpeg;base64,'.base64_encode($contents);

            
     $image = file_get_contents($jpgImage);

     $image = substr_replace($image, pack("cnn", 1, 300, 300), 13, 5);

     header("Content-type: image/jpeg");
     header('Content-Disposition: attachment; filename="'.basename($jpgImage).'"');
     echo  $image;
    
}

using this

 $image = substr_replace($image, pack("cnn", 1, 300, 300), 13, 5);

I want to increase dpi of image to 300 dpi.

I am unable to change the image dpi using these line of code

$jpgImage='data:image/jpeg;base64,'.base64_encode($contents);

$image = file_get_contents($jpgImage);

$image = substr_replace($image, pack("cnn", 1, 300, 300), 13, 5);

header("Content-type: image/jpeg");
header('Content-Disposition: attachment; filename="'.basename($jpgImage).'"');
echo  $image;

I used this link as reference Change image dpi using php

Answer 1

I would use imagemagic instead:

convert Bird.jpg -density 300 Bird2.jpg

But you could allso do that with GD.

Link to class

$filename = 'Bird.jpg';
$source = imagecreatefromjpeg($filename);
list($width, $height) = getimagesize($filename);

$b = new Resampler;
$im = $b->resample($source, $height, $width, 300);

file_put_contents('Bird2.jpg', $im);

Tested on Windows enviroment.

Class behind link

<?php
/**
 * Class used for resampling images
 */

class Resampler 
{   
    /**
     * Resample an image
     *
     * The image is returned as a string ready to be saved, it is not converteed back to a resource
     * as that would just be unnecessary 
     * 
     * @param  resource  $image Resource storing JPEG image
     * @param  integer $dpi     The dpi the image should be resampled at 
     * @return string           Resampled JPEG image as a string
     */
    function resample($image, $height, $width, $dpi = 300)
    {
        if(!$image)
        {
            throw new \Exception('Attempting to resample an empty image');
        }

        if(gettype($image) !== 'resource')
        {
            throw new \Exception('Attempting to resample something which is not a resource');
        }

        //Use truecolour image to avoid any issues with colours changing
        $tmp_img =  imagecreatetruecolor($width, $height);

        //Resample the image to be ready for print
        if(!imagecopyresampled ($tmp_img , $image , 0 , 0 ,0 , 0 , $width , $height , imagesx($image) , imagesy($image)))
        {
            throw new \Exception("Unable to resample image");
        }

        //Massive hack to get the image as a jpeg string but there is no php function which converts
        //a GD image resource to a JPEG string
        ob_start();
            imagejpeg($tmp_img, "", 100);
            $image = ob_get_contents();
        ob_end_clean();

        //change the JPEG header to 300 pixels per inch
        $image = substr_replace($image, pack("Cnn", 0x01, $dpi, $dpi), 13, 5);

        return $image;
    }
}

Answer 2

Simple Code to Re-generate image with adjustable quality.

function compress_image($source_url, $destination_url, $quality) {
    $info = getimagesize($source_url);
    $image = imagecreatefromjpeg($source_url);
    imagejpeg($image, $destination_url, $quality);
    return $destination_url;
}
echo compress_image($_FILES["file"]["tmp_name"], "destination .jpg", 80);//Adjust Quality

Note: Ensure that the GD library for PHP is installed.

Answer 3

After making some changes it worked for me.

public function pngtojpgAction()   
{
            //Code to convert png to jpg image
        $input = imagecreatefrompng($this->getRequest()->getParam('imagedata'));
        $width=imagesx($input);
        $height=imagesy($input);
        $output = imagecreatetruecolor($width, $height);
        $white = imagecolorallocate($output,  255, 255, 255);
        imagefilledrectangle($output, 0, 0, $width, $height, $white);
        imagecopy($output, $input, 0, 0, 0, 0, $width, $height);                  

        ob_start();
        imagejpeg($output);
        $contents =  ob_get_contents();
        //Converting Image DPI to 300DPI                
        $contents = substr_replace($contents, pack("cnn", 1, 300, 300), 13, 5);             
        ob_end_clean();     
        echo 'data:image/jpeg;base64,'.base64_encode($contents); 

}