CODEWITHSUNDEEP
pythonrastergdalsubtraction
ViktorG
ViktorG

Reputation: 515

subtract 2 pixel values from .tif file gives wrong result

I try to subtract two .tif images. For that I use the following code: 

import numpy as np
import os
from osgeo import gdal,ogr
import copy
from PIL import Image
import time

def loadTifAsArray(image, filepath):
    print("Loading "+image)
    path = os.path.join(filepath,image)
    tif = gdal.Open(path)
    tifArray = tif.ReadAsArray()
    return tifArray

def subtract(image1, image2):
    # Copy of one of the images is used for saving calculated values
    print("Subtracting ...")
    sub = copy.deepcopy(image1)
    rows = len(sub)    
    cols = len(sub[0])

    for px in range(cols):
        for py in range(rows):
            sub[px][py] = image1[px][py] - image2[px][py]

    return sub


start_time = time.time()

cwd = os.getcwd()
filepath = os.path.join(cwd,'tifs')
arr = os.listdir(filepath)
tifList = []

for image in arr:
    tifList.append(loadTifAsArray(image, filepath))
print("--- %s seconds for loading the images ---" % (time.time() - start_time))

sub = subtract(tifList[0], tifList[1])
print("--- %s seconds for loading and subtracting ---" % (time.time() - start_time))

I subtract the images, loaded as rasterdata, and simply make a deep copy of one of the images to store the calculated values in.

The problem is the calculated value. When I look at the values of both images at index [0][0], I get the following values: 

print(image1[0][0]) 
print(image2[0][0])

505

549

When I try to subtract them I get this: 

print(image1[0][0] - image2[0][0])

65492

I don't understand why that is and would appreciate any help!

Upvotes: 0

Views: 439

Answers (1)

ShlomiF
ShlomiF

Reputation: 2905

That smells like overflow! Basically, I'm assuming your images are uint16 images, and negative numbers "wrap around" back to the maximal value.
Note that you expected 44, but got 2^16 - 44.

The work-around is pretty simple; cast your images to float32 for instance, by adding at the beginning of your subtract function:

image1 = np.array(image1).astype(np.float32)
image2 = np.array(image2).astype(np.float32)

Good luck!

P.S: Of course np is just my own personal preference. You can cast in other ways that suit you best.

Upvotes: 2

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