Opencv + Python cv2.imwrite() and cv.imshow both images present different output -


i using opencv(3.0) , python 2.7 image processing, have issue cv2.imwrite() , cv2.imshow(). produce different output, code below:

tfinal = (255)*(nir_img-red_img)/(nir_img+red_img) cv2.imwrite(_db_img1+'_ndvi'+_ext_img,tfinal) cv2.imshow('ndvi',tfinal)

first image output of cv2.imshow()

cv2_imshow_op

second image output of cv2.imwrite()

cv2_imwrite_op

this can happen because of data type. imwrite , imshow determine data automatically, relaying on datatype. said in documentation imwrite , imshow:

imwrite:

the function imwrite saves image specified file. image format chosen based on filename extension (see imread() list of extensions). 8-bit (or 16-bit unsigned (cv_16u) in case of png, jpeg 2000, , tiff) single-channel or 3-channel (with ‘bgr’ channel order) images can saved using function.

imshow:

the function may scale image, depending on depth:

  • if image 8-bit unsigned, displayed is.
  • if image 16-bit unsigned or 32-bit integer, pixels divided 256. is, value range [0,255*256] mapped [0,255].
  • if image 32-bit floating-point, pixel values multiplied
    1. that is, value range [0,1] mapped [0,255].

so, seems underlying data type not unsigned char, float or 32-bit integer.

also because of operation priorities can run troubles with:

(255)*(nir_img-red_img)/(nir_img+red_img)

you can run overflow. better set values in range of [0; 1] , multiply them:

(255) * ( (nir_img-red_img)/(nir_img+red_img) )


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