feat: add vips straighten

Differences to pixbuf:
- plasma-straighten-positive-5 - MAE (Mean Average Error): 0.0117613
- plasma-straighten-on-vertical-image - MAE: 0.0159686
- plasma-straighten-negative-20 - MAE: 0.01176
Visually all the images appear to be shifted by 1px. Since I didn't spot any issues
with maths, I'm inclined to assume that it's due to rounding/trimming during computations.

CHANGELOG.md

@@ -10,6 +10,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 - Vips colour filters
 - Vips crop
 - Vips rotate
+- Vips straighten
 
 ### Removed
 - [BREAKING] dropped support for a broken 'dominant' border colour

lib/morandi/operation/vips_straighten.rb

@@ -0,0 +1,62 @@
+# frozen_string_literal: true
+
+require 'vips'
+
+module Morandi
+  module Operation
+    # Straighten operation
+    # Does a small (ie. not 90,180,270 deg) rotation and zooms to avoid cropping
+    # @!visibility private
+    class VipsStraighten < ImageOperation
+      # Colour for filling background post-rotation. It can bleed into the edge pixels during resize.
+      # Setting it to gray minimises the average impact
+      ROTATION_BACKGROUND_FILL_COLOUR = 127
+      ROTATION_BACKGROUND_FILL_ALPHA = 255
+
+      def self.rotation_background_fill_colour(channels_count:, alpha:)
+        return [ROTATION_BACKGROUND_FILL_COLOUR] * channels_count unless alpha # Eg [127, 127, 127] for RGB
+
+        # Eg [127, 127, 127, 255] for RGBA
+        ([ROTATION_BACKGROUND_FILL_COLOUR] * (channels_count - 1)) + [ROTATION_BACKGROUND_FILL_ALPHA]
+      end
+
+      attr_accessor :angle
+
+      def call(img)
+        return img if angle.zero?
+
+        original_width = img.width
+        original_height = img.height
+
+        # It is possible to first rotate, then fetch width/height of resulting image to calculate scale,
+        # but that would make us lose precision which degrades cropping accuracy
+        rotation_value_rad = angle * (Math::PI / 180)
+        post_rotation_bounding_box_width = (img.height.to_f * Math.sin(rotation_value_rad).abs) +
+                                           (img.width.to_f * Math.cos(rotation_value_rad).abs)
+        post_rotation_bounding_box_height = (img.width.to_f * Math.sin(rotation_value_rad).abs) +
+                                            (img.height.to_f * Math.cos(rotation_value_rad).abs)
+
+        # Calculate scaling required to fit the original width/height within rotated image without including background
+        scale = [post_rotation_bounding_box_width / original_width,
+                 post_rotation_bounding_box_height / original_height].max
+
+        background_fill_colour = self.class.rotation_background_fill_colour(channels_count: img.bands,
+                                                                            alpha: img.has_alpha?)
+        img = img.similarity(angle: angle, scale: scale, background: background_fill_colour)
+
+        # Better precision than img.width/img.height due to fractions preservation
+        post_scale_bounding_box_width = post_rotation_bounding_box_width * scale
+        post_scale_bounding_box_height = post_rotation_bounding_box_height * scale
+
+        width_diff = post_scale_bounding_box_width - original_width
+        height_diff = post_scale_bounding_box_height - original_height
+
+        # Round to nearest integer to reduce risk of ROTATION_BACKGROUND_FILL_COLOUR being visible in the corner
+        crop_x = (width_diff / 2).round
+        crop_y = (height_diff / 2).round
+
+        img.crop(crop_x, crop_y, original_width, original_height)
+      end
+    end
+  end
+end

lib/morandi/vips_image_processor.rb

@@ -3,6 +3,7 @@
 require 'vips'
 
 require 'morandi/srgb_conversion'
+require 'morandi/operation/vips_straighten'
 
 module Morandi
   # An alternative to ImageProcessor which is based on libvips for concurrent and less memory-intensive processing
@@ -97,6 +98,10 @@ def apply_rotate!
              else raise('"angle" option only accepts multiples of 90')
              end
 
+      unless @options['straighten'].to_f.zero?
+        @img = Morandi::Operation::VipsStraighten.new_from_hash(angle: @options['straighten'].to_f).call(@img)
+      end
+
       @image_width = @img.width
       @image_height = @img.height
     end

spec/morandi_spec.rb

@@ -239,7 +239,7 @@
       end
     end
 
-    describe 'when given a straighten option', vips_wip: processor_name == 'vips' do
+    describe 'when given a straighten option' do
       let(:options) { { 'straighten' => 5 } }
 
       it 'straightens images' do