Resizing Images with GD

Resize photographs by decoding the source, allocating the destination and resampling pixels.

What It Does and When to Use It

Resample an image; preserve source aspect ratio in real inputs. GD operates on raster images in PHP; Imagick delegates to ImageMagick; EXIF reads metadata and is not an image decoder.

Requirements, Inputs and Expected Behavior

Image decoding and encoding need the relevant GD, EXIF, or Imagick support; features vary by PHP build and server. Never upscale without reason; preserve alpha for PNG/WebP and verify the decoder returns GdImage.

Example 1: Basic Operation

Resample an image; preserve source aspect ratio in real inputs.

<?php
$source = imagecreatefromjpeg(__DIR__ . '/photo.jpg');
 if (!$source) throw new RuntimeException('Invalid JPEG');
 $target = imagecreatetruecolor(320, 180);
 imagecopyresampled($target, $source, 0, 0, 0, 0, 320, 180, imagesx($source), imagesy($source));
 imagejpeg($target, __DIR__ . '/small.jpg', 85);
 imagedestroy($source); imagedestroy($target);

Example 2: Behavior and Validation

Calculate dimensions without stretching small images or changing proportions.

<?php
[$width, $height] = getimagesize(__DIR__ . '/photo.jpg');
 $max = 600;
 $ratio = min(1, $max / max($width, $height));
 $newWidth = max(1, (int) round($width * $ratio));
 $newHeight = max(1, (int) round($height * $ratio));
 echo "$newWidth x $newHeight";

Practical Example: Application Use

Rescale a PNG and retain the alpha channel in the saved result.

<?php
$src = imagecreatefrompng(__DIR__ . '/source.png');
 $dst = imagescale($src, 240, -1, IMG_BICUBIC_FIXED);
 if (!$dst) throw new RuntimeException('Resize failed');
 imagealphablending($dst, false);
 imagesavealpha($dst, true);
 imagepng($dst, __DIR__ . '/scaled.png');
 imagedestroy($src); imagedestroy($dst);

Important Behavior and Edge Cases

The second example highlights: Calculate dimensions without stretching small images or changing proportions. The practical example adds: Rescale a PNG and retain the alpha channel in the saved result.

Comparison with Related Approaches

Common Beginner Mistakes

Key Takeaways

• Resize photographs by decoding the source, allocating the destination and resampling pixels.
• Rescale a PNG and retain the alpha channel in the saved result.
• Check image dimensions, true decoder availability, and actual output format rather than trusting a filename suffix.