Grayscale image not a jpeg

Question

I created a greyscale image like this

def create_new_image(size, luminance):
    width, height = size
    black_frame = int(luminance) * np.ones((width, height, 1), dtype=np.uint8)
    return black_frame

Where luminance is element of [0, 255]

I have saved the image using imageio

def save_image(image, output_path):
    imageio.imwrite(output_path, image)

Where the output_path is something like /valid_path/img.jpg

Now I want to load my grayscale image back:

img = imageio.imread(file, format ='jpg')

But what I get is a syntax error.

raise SyntaxError("not a JPEG file")
  File "<string>", line None
SyntaxError: not a JPEG file

If I don't specify the format, I get another error.

    "Could not find a format to read the specified file in %s mode" % modename
ValueError: Could not find a format to read the specified file in single-image mode

Why? Thanks

Answer 1

You can try :

def save_image(image, output_path):
    imageio.imwrite(output_path, format= "jpg", image)

to explicitly state that it is a jpg file.

Answer 2

JPEG files (compressed images) start with an image marker that always contains the marker code hex values FF D8 FF. It does not have a length of the file embedded, thus we need to find JPEG trailer, which is FF D9.

See the documentation using the link at this page.

As en example, opening a jpeg image with a hexadecimal viewer (for example Hex Viewer), you should see something like this:

Solution: In other words, try to add the header to the file before saving it as JPEG, you should solve your problem.

The page with the API's documentation can be found here. Following the doc, you should locate the right instruction that makes you specify the format for saving (as point out by @Meto in the answer).

Concluding: the solution is just specifying the format when you physically write the image in the hard disk:

imageio.imwrite(uri, im, format=None, **kwargs)

in your case format=jpg.

Moreover,

 imageio.show_formats()

Show a nicely formatted list of available formats.

Concluding, just try to replace

imageio.imwrite(output_path, image)

with

imageio.imwrite(output_path, image, format ='jpg' )

Please note that the solution is always the same in every answer. I have just added what happens specifying a format (i.e., just writes the right header).