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Why does the Sobel filter return a black square?

I'm trying to use sobel and prewitt filters from skimage for edge detection to compare the results, but for both I just get black squares!
That's my code:
import numpy as np
from skimage import filters
from PIL import Image
a=Image.open('F:/CT1.png').convert('L')
a.show()
a=np.asarray(a)
b=filters.sobel(a)
b=Image.fromarray(b)
b.show()

As most methods from scikit-image, the sobel function uses np.float64 for calculations, and thus converts your image appropriately to the range 0.0 ... 1.0. Following, your result b is also of type np.float64 with values in the same range. When now converting to some Pillow Image object, its mode is set to F, which is used for 32-bit floating point pixels.
Now, the documentation on Image.show tells us, for example:
On Windows, the image is opened with the standard PNG display utility.
It remains unclear, in which file format(?) the image is actually displayed. Seemingly, it's PNG, at least according to the temporary file name. But, for example, saving some Image object with mode F as PNG or JPG doesn't work! So, it seems, the image must be somehow converted to make it displayable. The first guess is, that some regular 8-bit image is chosen as default, since you get a nearly all black image, indicating that values 0 and maybe 1 are treated as "very dark". And, in fact, when using something like
b=Image.fromarray(b * 255)
the Windows image preview displays a proper image when using b.show().
So, that would be a workaround for the displaying.
Nevertheless, if you want to save the image instead, you don't necessarily need that conversion, but just need to use a proper file format to store those 32-bit information, TIFF for example:
b=Image.fromarray(b)
b.save('b.tiff')

OpenCV Python converting color-space image to YCbCr

I need convert an image from BGR to YCbCr in Python using OpenCV.
I have an image with size/resolution 512x512, but when the image is opened, the size is 128x128.
I'm doing:
image = cv2.imread(imageName, cv2.COLOR_BGR2YCR_CB)
Could anyone help me?

The problem:
If you look at the docs for imread, the function takes an integer flag called imreadmodes. This flag seems to accept information about resizing the image, rather than changing color spaces.
The solution:
I believe you are looking for the cv2.cvtColor function which uses a flag to determine the source and destination colorspaces.
Both flags are simple integer enumerations. I assume the imread function is simply doing the best it can with the wrong type of flag.
You probably want to do something like:
BGRImage = cv2.imread(imageName)
YCrCbImage = cv2.cvtColor(BGRImage, cv2.COLOR_BGR2YCR_CB)

OpenCV python canny Required argument 'threshold2' (pos 4) not found

I am trying to isolate text from an image with openCV before sending it to tesseract4 engine to maximize results.
I found this interesting post and I decided to copy the source and try by mysdelf
However I am getting issue with the first call to OpenCV
To reproduce:
Simply copy the code from the gist
launch command script.py /path/to/image.jpg
I am getting issue:
Required argument 'threshold2' (pos 4) not found
Do you maybe have an idea of what does it means.
I am a javascript, java and bash script developer but not python...
In a simple version:
import glob
import os
import random
import sys
import random
import math
import json
from collections import defaultdict
import cv2
from PIL import Image, ImageDraw
import numpy as np
from scipy.ndimage.filters import rank_filter
if __name__ == '__main__':
if len(sys.argv) == 2 and '*' in sys.argv[1]:
files = glob.glob(sys.argv[1])
random.shuffle(files)
else:
files = sys.argv[1:]
for path in files:
out_path = path.replace('.jpg', '.crop.png')
if os.path.exists(out_path): continue
orig_im = Image.open(path)
edges = cv2.Canny(np.asarray(orig_im), 100, 200)
Thanks in advance for your help

Edit: okay so this answer is apparently wrong, as I tried to send my own 16-bit int image into the function and couldn't reproduce the results.
Edit2: So I can reproduce the error with the following:
from PIL import Image
import numpy as np
import cv2
orig_im = Image.open('opencv-logo2.png')
threshold1 = 50
threshold2 = 150
edges = cv2.Canny(orig_im, 50, 100)
TypeError: Required argument 'threshold2' (pos 4) not found
So if the image was not cast to an array, i.e., the Image class was passed in, I get the error. The PIL Image class is a class with a lot of things other than the image data associated to it, so casting to a np.array is necessary to pass into functions. But if it was properly cast, everything runs swell for me.
In a chat with Dan Mašek, my idea below is a bit incorrect. It is true that the newer Canny() method needs 16-bit images, but the bindings don't look into the actual numpy dtype to see what bit-depth it is to decide which function call to use. Plus, if you try to actually send a uint16 image in, you get a different error:
edges = cv2.Canny(np.array([[0, 1234], [1234, 2345]], dtype=np.uint16), 50, 100)
error: (-215) depth == CV_8U in function Canny
So the answer I originally gave (below) is not the total culprit. Perhaps you accidentally removed the np.array() casting of the orig_im and got that error, or, something else weird is going on.
Original (wrong) answer
In OpenCV 3.2.0, a new method for Canny() was introduced to allow users to specify their own gradient image. In the original implementation, Canny() would use the Sobel() operator for calculating the gradients, but now you could calculate say the Scharr() derivatives and pass those into Canny() instead. So that's pretty cool. But what does this have to do with your problem?
The Canny() method is overloaded. And it decides which function you want to use based on the arguments you send in. The original call for Canny() with the required arguments looks like
cv2.Canny(image, threshold1, threshold2)
but the new overloaded method looks like
cv2.Canny(grad_x, grad_y, threshold1, threshold2)
Now, there was a hint in your error message:
Required argument 'threshold2' (pos 4) not found
Which one of these calls had threshold2 in position 4? The newer method call! So why was that being called if you only passed three args? Note that you were getting the error if you used a PIL image, but not if you used a numpy image. So what else made it assume you were using the new call?
If you check the OpenCV 3.3.0 Canny() docs, you'll see that the original Canny() call requires an 8-bit input image for the first positional argument, whereas the new Canny() call requires a 16-bit x derivative of input image (CV_16SC1 or CV_16SC3) for the first positional argument.
Putting two and two together, PIL was giving you a 16-bit input image, so OpenCV thought you were trying to call the new method.
So the solution here, if you wanted to continue using PIL, is to convert your image to an 8-bit representation. Canny() needs a single-channel (i.e. grayscale) image to run, first off. So you'll need to make sure the image is single-channel first, and then scale it and change the numpy dtype. I believe PIL will read a grayscale image as single channel (OpenCV by default reads all images as three-channel unless you tell it otherwise).
If the image is 16-bit, then the conversion is easy with numpy:
img = (img/256).astype('uint8')
This assumes img is a numpy array, so you would need to cast the PIL image to ndarray first with np.array() or np.asarray().
And then you should be able to run Canny() with the original function call.

The issue was coming from an incompatibility between interfaces used and openCV version.
I was using openCV 3.3 so the correct way to call it is:
orig_im = cv2.imread(path)
edges = cv2.Canny(orig_im, 100, 200)

Python Why does Image differ after tobytes() frombytes()?

I dont get why die image differ from each other after this 3 lines of code. In my opinion the images should be identical.
from PIL import Image
phone_img = Image.open("img2.png")
phone_img1 = Image.frombytes(phone_img.mode, phone_img.size, phone_img.tobytes())
phone_img1.save("img2_new.png","PNG")
img2.png: http://666kb.com/i/dk4ykapuzs4wc2e4g.png
img2_new.png: http://666kb.com/i/dk4ykz98cg97grxts.png

I'm not a big PIL/Pillow user, but:
You open your image with Image.open()
The returned object is of type Image
It holds more than the pure pixel-data (as you see by using .mode, .size)
You create a new image by interpreting the full object as pixel-data only!
The last part should probably something like frombytes(phone_img.mode, phone_img.size, phone_img.getdata())
Depending on the lib, one should take care of bit-mode too (8bit vs. 16bit for example)

Fill an image with pre-defined pattern with Wand

I have an image (like that: mask) and two integers, which represents a final image width & height. According to Wand's documentation Open empty image:
with Image(width=200, height=100) as img:
img.save(filename='200x100-transparent.png')
It will result in an empty image with transparent background.
Now, the question is: How to create a same empty image, but with mask image as background pattern?
The composite CLI command itself has a following operator:
-tile repeat composite operation across and down image
But how to achieve the same with Wand?

Well, after looking on ImageMagick's Composite source code itself, it became clear, that the Wand-driven solution should look like:
with Image(width=x, height=y) as img:
for x in xrange(0, img.width, crop_mask_path.width):
for y in xrange(0, img.height, crop_mask_path.height):
img.composite_channel('default_channels', crop_mask_path, 'over', x, y)
img.save(filename='patterned_image.png')

Building out the title iterator is the best solution in my opinion. However another hackish method would be to invoke the tile: protocol, and allow the internal ImageMagick methods to handle composites. You'll lose the control inherited by DIY, but gain some performance on optimized IM systems.
from wand.image import Image
from wand.api import library
with Image() as img:
# Same as `-size 400x400' needed by tile: protocol.
library.MagickSetOption(img.wand, 'size', '400x400')
# Prefix filename with `tile:' protocol.
img.read(filename='tile:rose.png')
img.save(filename='tile_rose.png')