Perceptual hash

pytest_mpl/kernels.py

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+"""
+This module contains the supported hashing kernel implementations.
+
+"""
+import hashlib
+from abc import ABC, abstractmethod
+
+import imagehash
+from PIL import Image
+
+#: The default hamming distance bit tolerance for "similar" imagehash hashes.
+DEFAULT_HAMMING_TOLERANCE = 2
+
+#: The default imagehash hash size (N), resulting in a hash of N**2 bits.
+DEFAULT_HASH_SIZE = 16
+
+#: Level of image detail (high) or structure (low) represented by phash .
+DEFAULT_HIGH_FREQUENCY_FACTOR = 4
+
+#: Registered kernel names.
+KERNEL_SHA256 = "sha256"
+KERNEL_PHASH = "phash"
+
+__all__ = [
+    "DEFAULT_HAMMING_TOLERANCE",
+    "DEFAULT_HASH_SIZE",
+    "DEFAULT_HIGH_FREQUENCY_FACTOR",
+    "KERNEL_PHASH",
+    "KERNEL_SHA256",
+    "KernelPHash",
+    "KernelSHA256",
+    "kernel_factory",
+]
+
+
+class Kernel(ABC):
+    """
+    Kernel abstract base class (ABC) which defines a simple common kernel API.
+
+    """
+
+    def __init__(self, plugin):
+        # Containment of the plugin allows the kernel to cherry-pick required state.
+        self._plugin = plugin
+
+    @abstractmethod
+    def equivalent_hash(self, result, baseline, marker=None):
+        """
+        Determine whether the kernel considers the provided result (actual)
+        and baseline (expected) hashes as similar.
+
+        Parameters
+        ----------
+        result : str
+            The hash of the image generated by the test.
+        baseline : str
+            The hash of the baseline image.
+        marker : pytest.Mark
+            The test marker, which may contain kwarg options to be
+            applied to the equivalence test.
+
+        Returns
+        -------
+        bool
+            Whether the result and baseline hashes are deemed similar.
+
+        """
+
+    @abstractmethod
+    def generate_hash(self, buffer):
+        """
+        Computes the hash of the image from the in-memory/open byte stream
+        buffer.
+
+        Parameters
+        ----------
+        buffer : stream
+            The in-memory/open byte stream of the image.
+
+        Returns
+        -------
+        str
+            The string representation (hexdigest) of the image hash.
+
+        """
+
+    def update_status(self, message):
+        """
+        Append the kernel status message to the provided message.
+
+        Parameters
+        ----------
+        message : str
+            The existing status message.
+
+        Returns
+        -------
+        str
+            The updated status message.
+
+        """
+        return message
+
+    def update_summary(self, summary):
+        """
+        Refresh the image comparison summary with relevant kernel entries.
+
+        Parameters
+        ----------
+        summary : dict
+            Image comparison test report summary.
+
+        Returns
+        -------
+        None
+
+        """
+        # The "name" class property *must* be defined in derived child class.
+        summary["kernel"] = self.name
+
+
+class KernelPHash(Kernel):
+    """
+    Kernel that calculates a perceptual hash of an image for the
+    specified hash size (N) and high frequency factor.
+
+    Where the resultant perceptual hash will be composed of N**2 bits.
+
+    """
+
+    name = KERNEL_PHASH
+
+    def __init__(self, plugin):
+        super().__init__(plugin)
+        # Keep state of the equivalence result.
+        self.equivalent = None
+        # Keep state of hash hamming distance (whole number) result.
+        self.hamming_distance = None
+        # Value may be overridden by py.test marker kwarg.
+        self.hamming_tolerance = (
+            self._plugin.hamming_tolerance or DEFAULT_HAMMING_TOLERANCE
+        )
+        # The hash-size (N) defines the resultant N**2 bits hash size.
+        self.hash_size = self._plugin.hash_size
+        # The level of image detail (high freq) or structure (low freq)
+        # represented in perceptual hash thru discrete cosine transform.
+        self.high_freq_factor = (
+            self._plugin.high_freq_factor or DEFAULT_HIGH_FREQUENCY_FACTOR
+        )
+        # py.test marker kwarg.
+        self.option = "hamming_tolerance"
+
+    def equivalent_hash(self, result, baseline, marker=None):
+        if marker:
+            value = marker.kwargs.get(self.option)
+            if value is not None:
+                # Override with the decorator marker value.
+                self.hamming_tolerance = int(value)
+        # Convert string hexdigest hashes to imagehash.ImageHash instances.
+        result = imagehash.hex_to_hash(result)
+        baseline = imagehash.hex_to_hash(baseline)
+        # Unlike cryptographic hashes, perceptual hashes can measure the
+        # degree of "similarity" through hamming distance bit differences
+        # between the hashes.
+        self.hamming_distance = result - baseline
+        self.equivalent = self.hamming_distance <= self.hamming_tolerance
+        return self.equivalent
+
+    def generate_hash(self, buffer):
+        buffer.seek(0)
+        data = Image.open(buffer)
+        phash = imagehash.phash(
+            data, hash_size=self.hash_size, highfreq_factor=self.high_freq_factor
+        )
+        return str(phash)
+
+    def update_status(self, message):
+        result = str() if message is None else str(message)
+        # Only update the status message for non-equivalent hash comparisons.
+        if self.equivalent is False:
+            msg = (
+                f"Hash hamming distance of {self.hamming_distance} bits > "
+                f"hamming tolerance of {self.hamming_tolerance} bits."
+            )
+            result = f"{message} {msg}" if len(result) else msg
+        return result
+
+    def update_summary(self, summary):
+        super().update_summary(summary)
+        summary["hamming_distance"] = self.hamming_distance
+        summary["hamming_tolerance"] = self.hamming_tolerance
+
+
+class KernelSHA256(Kernel):
+    """
+    A simple kernel that calculates a 256-bit cryptographic SHA hash
+    of an image.
+
+    """
+
+    name = KERNEL_SHA256
+
+    def equivalent_hash(self, result, baseline, marker=None):
+        # Simple cryptographic hash binary comparison. Interpretation of
+        # the comparison result is that the hashes are either identical or
+        # not identical. For non-identical hashes, it is not possible to
+        # determine a heuristic of hash "similarity" due to the nature of
+        # cryptographic hashes.
+        return result == baseline
+
+    def generate_hash(self, buffer):
+        buffer.seek(0)
+        data = buffer.read()
+        hasher = hashlib.sha256()
+        hasher.update(data)
+        return hasher.hexdigest()
+
+
+#: Registry of available hashing kernel factories.
+kernel_factory = {
+    KernelPHash.name: KernelPHash,
+    KernelSHA256.name: KernelSHA256,
+}