[newFeature] setOpacity processor (#79)

* [newFeature] setOpacity processor

Processor used to set the max alpha value of a picture.
- processor added to base.py
- test added to test_processors.py

Author: Psylocibus

pilkit/processors/base.py

@@ -223,3 +223,46 @@ def process(self, img):
         new_img = Image.new('RGBA', img.size, self.background_color)
         new_img.paste(img, img)
         return new_img
+
+class SetOpacity(object):
+    """
+    Convert an image to RGBA mode, retrive the maximum opacity of the image and set this maximum opacity to a given value.
+    - On a RGB image, the opacity will be even.
+    - On a RGBA image, the pixel with the higher alpha value will be set to the given value. 
+    The other values will be linearly interpolated.
+    """
+
+    def __init__(self, max_alpha = 255):
+        """
+        :param max_alpha: integer between (0,255) representing the highest value wanted for the alpha chanel
+        :param interpolation: interpolation type (only "LINEAR" available for now)
+        """
+        #check values
+        if type(max_alpha) != int :
+            raise TypeError("max_alpha shall be an integer")
+        if max_alpha < 0 or max_alpha > 255 :
+            raise ValueError("max_alpha shall be an integer between [0;255]")
+        
+        #init
+        self.max_alpha = max_alpha
+
+    def interpolate(self, x, max_y, max_x):
+        y = float(x)*float(max_y)/float(max_x)
+        return int(y)
+
+    def process(self, img):
+        #convert to RGBA
+        new_img = img.convert("RGBA")
+        
+        #split chanels
+        r,g,b,a = new_img.split()
+
+        #retrive max alpha value
+        max_alpha_original = a.getextrema()[1]
+
+        #interpolate each pixel
+        for y in range(new_img.height):
+            for x in range(new_img.width):
+                a.putpixel((x,y), self.interpolate(a.getpixel((x,y)), self.max_alpha, max_alpha_original))
+
+        return Image.merge("RGBA",(r,g,b,a))

tests/assets/multiple_alpha_image.png

@@ -4,7 +4,7 @@
 
 from pilkit.lib import Image, ImageDraw, ImageColor
 from pilkit.processors import (Resize, ResizeToFill, ResizeToFit, SmartCrop,
-                               SmartResize, MakeOpaque, ColorOverlay, Convert,
+                               SmartResize, MakeOpaque, SetOpacity, ColorOverlay, Convert,
                                GaussianBlur, ImageOverlay)
 from pilkit.processors.resize import Thumbnail
 from .utils import create_image, compare_images, get_image_file
@@ -206,3 +206,57 @@ def test_make_gifs_opaque():
     path = os.path.join(dir, 'assets', 'cat.gif')
     gif = Image.open(path)
     MakeOpaque().process(gif)
+
+def test_setOpacity():
+    #test on RGB image
+    ref_1 = Image.open(get_image_file("reference.png")).convert("RGB")
+    nb_pixels = ref_1.width*ref_1.height
+    result_1 = SetOpacity(0).process(ref_1)
+    assert result_1.mode == "RGBA" #mode of the output is RGBA
+    expected_histogram_1 = [0]*256 #initialize an empty histogram
+    expected_histogram_1[0] = nb_pixels #all pixels at 0
+    r,g,b,a = result_1.split()
+    assert a.histogram() == expected_histogram_1 #opacity is 0 on every pixel
+
+    result_2 = SetOpacity(50).process(ref_1)
+    assert result_2.mode == "RGBA" #mode of the output is RGBA
+    expected_histogram_2 = [0]*256 #initialize an empty histogram
+    expected_histogram_2[50] = nb_pixels #all pixels at 50
+    r,g,b,a = result_2.split()
+    assert a.histogram() == expected_histogram_2 #opacity is 0 on every pixel
+
+    result_3 = SetOpacity(125).process(ref_1)
+    assert result_3.mode == "RGBA" #mode of the output is RGBA
+    expected_histogram_3 = [0]*256 #initialize an empty histogram
+    expected_histogram_3[125] = nb_pixels #all pixels at 125
+    r,g,b,a = result_3.split()
+    assert a.histogram() == expected_histogram_3 #opacity is 0 on every pixel
+
+    #test on RGBA image
+    ref_2 = Image.open(get_image_file("multiple_alpha_image.png")) #image 60x30px with following alpha values [0, 8, 32, 78, 147, 244] (300px for each values)
+    result_4 = SetOpacity(244).process(ref_2) #no modification
+    assert compare_images(result_4,ref_2)
+
+    result_5 = SetOpacity(100).process(ref_2) #no modification
+    expected_histogram_5 = [0]*256 #initialize an empty histogram
+    expected_histogram_5[0] = 300 #all px @0 stay @0
+    expected_histogram_5[3] = 300 #px @8 lower to @3
+    expected_histogram_5[13] = 300 #px @32 lower to @13
+    expected_histogram_5[31] = 300 #px @78 lower to @31
+    expected_histogram_5[60] = 300 #px @147 lower to @60
+    expected_histogram_5[100] = 300 #px @244 lower to @100 (as wanted)
+    r,g,b,a = result_5.split()
+    assert a.histogram() == expected_histogram_5
+
+    # test error
+    with pytest.raises(ValueError) :
+        SetOpacity(-1).process(ref2) #Value error
+    
+    with pytest.raises(ValueError) :
+        SetOpacity(256).process(ref2) #Value error
+    
+    with pytest.raises(TypeError) :
+        SetOpacity(0.1).process(ref2) #Type error
+    
+    with pytest.raises(TypeError) :
+        SetOpacity("255").process(ref2) #Type error