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POConvert RGBA PNG to RGB with PIL
 

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  1. POConvert RGBA PNG to RGB with PIL
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    <p>I'm using PIL to convert a transparent PNG image uploaded with Django to a JPG file. The output looks broken.</p> <h3>Source file</h3> <p><img src="https://i.stack.imgur.com/I2uNe.png" alt="transparent source file"></p> <h3>Code</h3> <pre><code>Image.open(object.logo.path).save('/tmp/output.jpg', 'JPEG') </code></pre> <p>or</p> <pre><code>Image.open(object.logo.path).convert('RGB').save('/tmp/output.png') </code></pre> <h3>Result</h3> <p>Both ways, the resulting image looks like this:</p> <p><img src="https://i.stack.imgur.com/rmlPz.png" alt="resulting file"></p> <p>Is there a way to fix this? I'd like to have white background where the transparent background used to be.</p> <hr> <h1>Solution</h1> <p>Thanks to the great answers, I've come up with the following function collection:</p> <pre><code>import Image import numpy as np def alpha_to_color(image, color=(255, 255, 255)): """Set all fully transparent pixels of an RGBA image to the specified color. This is a very simple solution that might leave over some ugly edges, due to semi-transparent areas. You should use alpha_composite_with color instead. Source: http://stackoverflow.com/a/9166671/284318 Keyword Arguments: image -- PIL RGBA Image object color -- Tuple r, g, b (default 255, 255, 255) """ x = np.array(image) r, g, b, a = np.rollaxis(x, axis=-1) r[a == 0] = color[0] g[a == 0] = color[1] b[a == 0] = color[2] x = np.dstack([r, g, b, a]) return Image.fromarray(x, 'RGBA') def alpha_composite(front, back): """Alpha composite two RGBA images. Source: http://stackoverflow.com/a/9166671/284318 Keyword Arguments: front -- PIL RGBA Image object back -- PIL RGBA Image object """ front = np.asarray(front) back = np.asarray(back) result = np.empty(front.shape, dtype='float') alpha = np.index_exp[:, :, 3:] rgb = np.index_exp[:, :, :3] falpha = front[alpha] / 255.0 balpha = back[alpha] / 255.0 result[alpha] = falpha + balpha * (1 - falpha) old_setting = np.seterr(invalid='ignore') result[rgb] = (front[rgb] * falpha + back[rgb] * balpha * (1 - falpha)) / result[alpha] np.seterr(**old_setting) result[alpha] *= 255 np.clip(result, 0, 255) # astype('uint8') maps np.nan and np.inf to 0 result = result.astype('uint8') result = Image.fromarray(result, 'RGBA') return result def alpha_composite_with_color(image, color=(255, 255, 255)): """Alpha composite an RGBA image with a single color image of the specified color and the same size as the original image. Keyword Arguments: image -- PIL RGBA Image object color -- Tuple r, g, b (default 255, 255, 255) """ back = Image.new('RGBA', size=image.size, color=color + (255,)) return alpha_composite(image, back) def pure_pil_alpha_to_color_v1(image, color=(255, 255, 255)): """Alpha composite an RGBA Image with a specified color. NOTE: This version is much slower than the alpha_composite_with_color solution. Use it only if numpy is not available. Source: http://stackoverflow.com/a/9168169/284318 Keyword Arguments: image -- PIL RGBA Image object color -- Tuple r, g, b (default 255, 255, 255) """ def blend_value(back, front, a): return (front * a + back * (255 - a)) / 255 def blend_rgba(back, front): result = [blend_value(back[i], front[i], front[3]) for i in (0, 1, 2)] return tuple(result + [255]) im = image.copy() # don't edit the reference directly p = im.load() # load pixel array for y in range(im.size[1]): for x in range(im.size[0]): p[x, y] = blend_rgba(color + (255,), p[x, y]) return im def pure_pil_alpha_to_color_v2(image, color=(255, 255, 255)): """Alpha composite an RGBA Image with a specified color. Simpler, faster version than the solutions above. Source: http://stackoverflow.com/a/9459208/284318 Keyword Arguments: image -- PIL RGBA Image object color -- Tuple r, g, b (default 255, 255, 255) """ image.load() # needed for split() background = Image.new('RGB', image.size, color) background.paste(image, mask=image.split()[3]) # 3 is the alpha channel return background </code></pre> <h2>Performance</h2> <p>The simple non-compositing <code>alpha_to_color</code> function is the fastest solution, but leaves behind ugly borders because it does not handle semi transparent areas.</p> <p>Both the pure PIL and the numpy compositing solutions give great results, but <code>alpha_composite_with_color</code> is much faster (8.93 msec) than <code>pure_pil_alpha_to_color</code> (79.6 msec). <del>If numpy is available on your system, that's the way to go.</del> (Update: The new pure PIL version is the fastest of all mentioned solutions.)</p> <pre><code>$ python -m timeit "import Image; from apps.front import utils; i = Image.open(u'logo.png'); i2 = utils.alpha_to_color(i)" 10 loops, best of 3: 4.67 msec per loop $ python -m timeit "import Image; from apps.front import utils; i = Image.open(u'logo.png'); i2 = utils.alpha_composite_with_color(i)" 10 loops, best of 3: 8.93 msec per loop $ python -m timeit "import Image; from apps.front import utils; i = Image.open(u'logo.png'); i2 = utils.pure_pil_alpha_to_color(i)" 10 loops, best of 3: 79.6 msec per loop $ python -m timeit "import Image; from apps.front import utils; i = Image.open(u'logo.png'); i2 = utils.pure_pil_alpha_to_color_v2(i)" 10 loops, best of 3: 1.1 msec per loop </code></pre>
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    1. COFor a bit more speed, I believe `im = image.copy()` can be removed from `pure_pil_alpha_to_color_v2` without changing the result. (After changing subsequent instances of `im` to `image`, of course.)
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    2. CO@unutbu ah, of course :) thanks.
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