feat: Add ability to define pixel difference calculators (#17)

README.md

@@ -89,3 +89,52 @@ $this->assertThat(
     $this->isSimilarGD('./tests/expected.png')
 );
 ```
+
+## Difference calculation
+
+By default, this library calculates the difference between two images by
+comparing the RGBA color channel information at each pixel coordinate of the
+source image and the test image, and averaging the difference between each
+pixel to calculate the difference score.
+
+This will work for the majority of cases, but may give incorrect scoring 
+in certain circumstances, such as images that contain a lot of transparency.
+
+An alternative calculation method, which scales the RGB color channels
+based on their alpha transparency - meaning more transparent pixels will
+affect the difficulty score less to offset their less observable difference
+on the image itself - can be enabled by adding a new `ScaledRgbChannels`
+instance to the 5th parameter of the `assertSimilarGD` or `assertNotSimilarGD`
+methods.
+
+```php
+use AssertGD\DiffCalculator\ScaledRgbChannels;
+
+public function testImage()
+{
+    $this->assertSimilarGD(
+        'expected.png',
+        'actual.png',
+        '',
+        0,
+        new ScaledRgbChannels()
+    );
+}
+```
+
+### Custom difference calculators
+
+If you wish to completely customise how calculations are done in this
+library, you may also create your own calculation algorithm by creating
+a class that implements the `AssertGd\DiffCalculator` interface.
+
+A class implementing this interface must provide a `calculate` method
+that is provided two `GdImage` instances, and the X and Y co-ordinate
+(as `ints`) of the pixel being compared in both images.
+
+The method should return a `float` between `0` and `1`, where 0 is
+an exact match and 1 is the complete opposite.
+
+You may then provide an instance of the class as the 5th parameter of
+the `assertSimilarGD` or `assertNotSimilarGD` method to use this
+calculation method for determining the image difference.

src/DiffCalculator.php

@@ -0,0 +1,24 @@
+<?php
+
+namespace AssertGD;
+
+use AssertGD\GDImage;
+
+/**
+ * Difference calculator.
+ *
+ * Determines the difference between two given images.
+ */
+interface DiffCalculator
+{
+    /**
+     * Calculates the difference between two pixels at the given coordinates.
+     *
+     * This method will be provided with two `GDImage` objects representing the images being compared, and co-ordinates
+     * of the pixel being compared.
+     *
+     * The method should return a float value between 0 and 1 inclusive, with 0 meaning that the pixels of both images
+     * at the given co-ordinates are an exact match, and 1 meaning that the pixels are the complete opposite.
+     */
+    public function calculate(GDImage $imageA, GDImage $imageB, int $pixelX, int $pixelY): float;
+}

src/DiffCalculator/RgbaChannels.php

@@ -0,0 +1,31 @@
+<?php
+
+namespace AssertGD\DiffCalculator;
+
+use AssertGD\DiffCalculator;
+use AssertGD\GDImage;
+
+/**
+ * Calculate the difference between two pixels using the RGBA channels.
+ *
+ * This is the default calculation method used by the `AssertGD` package. It simply takes each individual channel and
+ * compares the delta between the channel values of the two images.
+ *
+ * This works well for most images, but may not work for images with transparent pixels if the transparent pixels have
+ * different RGB values.
+ */
+class RgbaChannels implements DiffCalculator
+{
+    public function calculate(GDImage $imageA, GDImage $imageB, int $pixelX, int $pixelY): float
+    {
+        $pixelA = $imageA->getPixel($pixelX, $pixelY);
+        $pixelB = $imageB->getPixel($pixelX, $pixelY);
+
+        $diffR = abs($pixelA['red'] - $pixelB['red']) / 255;
+        $diffG = abs($pixelA['green'] - $pixelB['green']) / 255;
+        $diffB = abs($pixelA['blue'] - $pixelB['blue']) / 255;
+        $diffA = abs($pixelA['alpha'] - $pixelB['alpha']) / 127;
+
+        return ($diffR + $diffG + $diffB + $diffA) / 4;
+    }
+}

src/DiffCalculator/ScaledRgbChannels.php

@@ -0,0 +1,45 @@
+<?php
+
+namespace AssertGD\DiffCalculator;
+
+use AssertGD\DiffCalculator;
+use AssertGD\GDImage;
+
+/**
+ * Calculate the difference between two pixels using the RGB channels, and scales down the RGB difference by the alpha
+ * channel.
+ *
+ * This calculation will pre-multiply the RGB channels by the opacity percentage (alpha) of the pixel, meaning that a
+ * translucent pixel will have less of an impact on the overall difference than an opaque pixel. For transparent pixels,
+ * this will mean that the RGB difference will be scaled down to zero, effectively meaning that transparent pixels will
+ * match regardless of their RGB values.
+ *
+ * This calculation method is useful for images with transparent pixels or images that have been anti-aliased or
+ * blurred over a transparent background, effectively making translucent pixels less likely to cause a false positive as
+ * being different.
+ */
+class ScaledRgbChannels implements DiffCalculator
+{
+    public function calculate(GDImage $imageA, GDImage $imageB, int $pixelX, int $pixelY): float
+    {
+        $pixelA = $this->premultiply($imageA->getPixel($pixelX, $pixelY));
+        $pixelB = $this->premultiply($imageB->getPixel($pixelX, $pixelY));
+
+        $diffR = abs($pixelA['red'] - $pixelB['red']) / 255;
+        $diffG = abs($pixelA['green'] - $pixelB['green']) / 255;
+        $diffB = abs($pixelA['blue'] - $pixelB['blue']) / 255;
+
+        return ($diffR + $diffG + $diffB) / 4;
+    }
+
+    protected function premultiply(array $pixel)
+    {
+        $alpha = 1 - ($pixel['alpha'] / 127);
+
+        return [
+            'red' => $pixel['red'] * $alpha,
+            'green' => $pixel['green'] * $alpha,
+            'blue' => $pixel['blue'] * $alpha,
+        ];
+    }
+}

src/GDAssertTrait.php

@@ -2,12 +2,19 @@
 
 namespace AssertGD;
 
+use AssertGD\DiffCalculator\RgbaChannels;
+
 /**
  * Use this trait in a test case class to gain access to the image similarity
  * assertions.
  */
 trait GDAssertTrait
 {
+    /**
+     * @var DiffCalculator The difference calculator to compare the images with.
+     */
+    protected $diffCalculator;
+
     /**
      * Asserts that the difference between $expected and $actual is AT MOST
      * $threshold. $expected and $actual can be GD image resources or paths to
@@ -22,14 +29,15 @@ trait GDAssertTrait
      * @param string|resource $actual    The actual image.
      * @param string          $message   The failure message.
      * @param float           $threshold Error threshold between 0 and 1.
+     * @param DiffCalculator|null     $diffCalculator The difference calculator to use.
      *
      * @return void
      *
      * @throws PHPUnit\Framework\AssertionFailedError
      */
-    public function assertSimilarGD($expected, $actual, $message = '', $threshold = 0)
+    public function assertSimilarGD($expected, $actual, $message = '', $threshold = 0, $diffCalculator = null)
     {
-        $constraint = $this->isSimilarGD($expected, $threshold);
+        $constraint = $this->isSimilarGD($expected, $threshold, $diffCalculator);
         $this->assertThat($actual, $constraint, $message);
     }
 
@@ -44,15 +52,16 @@ public function assertSimilarGD($expected, $actual, $message = '', $threshold =
      * @param string|resource $actual    The actual image.
      * @param string          $message   The failure message.
      * @param float           $threshold Error threshold between 0 and 1.
+     * @param DiffCalculator|null     $diffCalculator The difference calculator to use.
      *
      * @return void
      *
      * @throws PHPUnit\Framework\AssertionFailedError
      */
-    public function assertNotSimilarGD($expected, $actual, $message = '', $threshold = 0)
+    public function assertNotSimilarGD($expected, $actual, $message = '', $threshold = 0, $diffCalculator = null)
     {
         $constraint = $this->logicalNot(
-            $this->isSimilarGD($expected, $threshold)
+            $this->isSimilarGD($expected, $threshold, $diffCalculator)
         );
         $this->assertThat($actual, $constraint, $message);
     }
@@ -66,11 +75,28 @@ public function assertNotSimilarGD($expected, $actual, $message = '', $threshold
      *
      * @param string|resource $expected  The expected image.
      * @param float           $threshold Error threshold between 0 and 1.
+     * @param DiffCalculator|null     $diffCalculator The difference calculator to use.
      *
      * @return GDSimilarityConstraint The constraint.
      */
-    public function isSimilarGD($expected, $threshold = 0)
+    public function isSimilarGD($expected, $threshold = 0, $diffCalculator = null)
     {
-        return new GDSimilarityConstraint($expected, $threshold);
+        return new GDSimilarityConstraint($expected, $threshold, $diffCalculator ?? $this->diffCalculator ?? new RgbaChannels());
+    }
+
+    /**
+     * Sets the difference calculator to use for image comparisons in this test case.
+     *
+     * @var DiffCalculator $diffCalculator
+     */
+    public function setDiffCalculator($diffCalculator): void
+    {
+        if (!($diffCalculator instanceof DiffCalculator)) {
+            throw new \InvalidArgumentException(
+                'The difference calculator must implement the `AssertGD\DiffCalculator` interface'
+            );
+        }
+
+        $this->diffCalculator = $diffCalculator;
     }
 }

src/GDSimilarityConstraint.php

@@ -2,6 +2,7 @@
 
 namespace AssertGD;
 
+use AssertGD\DiffCalculator\RgbaChannels;
 use PHPUnit\Framework\Constraint\Constraint;
 
 /**
@@ -16,18 +17,24 @@ class GDSimilarityConstraint extends Constraint
 {
     private $expected;
     private $threshold;
+    /**
+     * @var DiffCalculator The difference calculator to compare the images with.
+     */
+    private $diffCalculator;
 
     /**
      * Constructs a new constraint. A threshold of 0 means only exactly equal
      * images are allowed, while a threshold of 1 matches every image.
      *
      * @param string|resource $expected  File name or resource to match against.
      * @param float           $threshold Error threshold between 0 and 1.
+     * @param DiffCalculator|null     $diffCalculator The difference calculator to use.
      */
-    public function __construct($expected, $threshold = 0)
+    public function __construct($expected, $threshold = 0, $diffCalculator = null)
     {
         $this->expected = $expected;
         $this->threshold = $threshold;
+        $this->diffCalculator = $diffCalculator ?? new RgbaChannels();
     }
 
     /**
@@ -64,7 +71,7 @@ public function matches($other): bool
         $delta = 0;
         for ($x = 0; $x < $w; ++$x) {
             for ($y = 0; $y < $h; ++$y) {
-                $delta += $this->getPixelError($imgExpec, $imgOther, $x, $y);
+                $delta += $this->diffCalculator->calculate($imgExpec, $imgOther, $x, $y);
             }
         }
 
@@ -76,27 +83,4 @@ public function matches($other): bool
 
         return $error <= $this->threshold;
     }
-
-    /**
-     * Calculates the error between 0 and 1 (inclusive) of a specific pixel.
-     *
-     * @param GDImage $imgA The first image.
-     * @param GDImage $imgB The second image.
-     * @param int     $x    The pixel's x coordinate.
-     * @param int     $y    The pixel's y coordinate.
-     *
-     * @return float The pixel error.
-     */
-    private function getPixelError(GDImage $imgA, GDImage $imgB, $x, $y)
-    {
-        $pixelA = $imgA->getPixel($x, $y);
-        $pixelB = $imgB->getPixel($x, $y);
-
-        $diffR = abs($pixelA['red'] - $pixelB['red']) / 255;
-        $diffG = abs($pixelA['green'] - $pixelB['green']) / 255;
-        $diffB = abs($pixelA['blue'] - $pixelB['blue']) / 255;
-        $diffA = abs($pixelA['alpha'] - $pixelB['alpha']) / 127;
-
-        return ($diffR + $diffG + $diffB + $diffA) / 4;
-    }
 }

tests/GDAssertTraitTest.php

@@ -1,5 +1,6 @@
 <?php
 
+use AssertGD\DiffCalculator\ScaledRgbChannels;
 use PHPUnit\Framework\TestCase;
 
 use AssertGD\GDAssertTrait;
@@ -49,4 +50,23 @@ public function testJpeg()
         $this->assertSimilarGD('./tests/images/jpeg.jpg',
             './tests/images/jpeg-alt.jpg', '', 0.1);
     }
+
+    public function testAlternativeDiffCalculator()
+    {
+        // the default method of calculating images will not consider these images exact due to the transparent pixels
+        // having different RGB values
+        $this->assertNotSimilarGD('./tests/images/transparent-black.gif', './tests/images/transparent-white.gif');
+
+        // using the ScaledRgbChannels diff calculator, the images will be considered exact
+        $this->assertSimilarGD('./tests/images/transparent-black.gif', './tests/images/transparent-white.gif',
+             '', 0, new ScaledRgbChannels());
+    }
+
+    public function testSetDiffCalculator()
+    {
+        // apply diff calculator on all further assertions
+        $this->setDiffCalculator(new ScaledRgbChannels());
+
+        $this->assertSimilarGD('./tests/images/transparent-black.gif', './tests/images/transparent-white.gif');
+    }
 }