CODEWITHSUNDEEP
pythonopencv
Arthur Le Calvez
Arthur Le Calvez

Reputation: 433

Open CV cv.imread modifying image

I use cv.imread to read a png file in python. When I then use cv.imwrite function to immediately save the image i then find that the colours in the image have changed slightly. I am trying to perform character recognition on this image and the OCR performs far less well on the image in python than the original image. The first image is the original, and the second is the saved one with OpenCV.

First

second

We can see that the green has changed slightly and whilst this does not seem important it affects the OCR and I therefore imagine that other changes are happening to the png. Does anyone know why this might be and how i can resolve this.

The code is as follows 

img = cv2.imread('file.png')
cv2.imwrite('out.png', img)

When I run file.png in tesseract for character recognition I get great results but when I run out.png in tesseract far less words get recognised correctly.

Upvotes: 2

Views: 4159

Answers (1)

Jeru Luke
Jeru Luke

Reputation: 21203

When you have a .png image file you ought to read as a .png file.

I downloaded your image and did some analysis myself.

  • First, I read the image as you did:

    img = cv2.imread('file.png')

    img.shape returns (446, 864, 3) i.e an image with 3 channels.

  • Next I read the same image using cv2.IMREAD_UNCHANGED:

    img = cv2.imread('file.png', cv2.IMREAD_UNCHANGED)

    img.shape returns (446, 864, 4) i.e an image with 4 channels.

.png files have an additional transparency channel. So next you come accross a .png file read it using cv2.IMREAD_UNCHANGED flag

UPDATE:

Enlisting the various ways to read an image:

for var in dir(cv2):
    if var.startswith('IMREAD'):
        print(var)

returns:

IMREAD_ANYCOLOR
IMREAD_ANYDEPTH
IMREAD_COLOR
IMREAD_GRAYSCALE
IMREAD_LOAD_GDAL
IMREAD_UNCHANGED

Upvotes: 4

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