I am implementing lz77 compression for images. But if I insert an RGB image the output is wrong could any of you help me?

Question

when i insert a grayscale image then the algorithm works if i insert a rgb image the output is wrong. is the way I work with the rgb image correct? I don't understand why can someone help me?

library used

from PIL import Image

import numpy as np from cv2 import cv2

 def lz77Compress (image,sw,lab):
    img = cv2.imread(image)
    print("Initial size", img)
    flat = np.array(img).flatten()
    #cv2.imshow("input", img)
    row = img.shape[0]
    col = img.shape[1]
    tot = row * col
    slidingWindows = sw
    lookAhead = lab

array of tuyple and charactert

encodedTuple = np.array([]) 
encodedChar = np.array([])

pointer in the search buffer

sbPointer = 0
while sbSize + sbPointer < tot :
    max_match = 0
    max_match_go_back = 0        
    selChar = sbPointer + sbSize

empty sequence

   seqY = np.array([])
    for i in range(sbPointer,sbPointer + sbSize):
        if(flat[i] == encodeCharacters):
            seqY = np.append(seqY,i)

check if there is a match in the lookAheadBuffer

 if(seqY.size == 0 ):
        encodedTuple = np.append(encodedTuple,(0,0))
        encodedChar = np.append (encodedChar,encodeCharacters)
    else:

        for j in seqY:
        #lunghezza della corrispodenza
            matchLenght= 0
            returnBack = selChar -i
            it = 0 
            while selChar + it < tot :
                if flat[it + j] == flat[selChar + it]:
                    matchLenght +=1
                    it +=1
              # se non trova corrispondenze
                else: 
                    break
            if matchLenght>max_match:
               max_match = matchLenght
               returnBack = max_match_go_back

save corrispondence and tuple in array

encodedTuple = np.append(encodedTuple,(max_match_go_back,max_match))
encodedChar = np.append(encodedChar,flat[selChar + max_match - 1])
        
       sbPointer+= 1 +max_match

**save of encodedTuple, encodedChar , file compresses.txt and imsize of decompression **

print("Prova", encodedTuple, encodedChar)
print("ArrayBin",encodedTuple.tolist(), encodedChar.tolist())
np.save("encodedTuple", encodedTuple) 
np.save("encodedChar", encodedChar)
a = encodedTuple.tolist()
b = encodedChar.tolist()
c= (a,b)
print("File compresso in : Copressed.txt")
output = open("Compressed.txt","w+")
output.write(str(c))
imgSize = open('imgSize.txt', "w")
imgSize.write(str(row) + '\n')  # write row dimension
imgSize.write(str(col) + '\n')  # write col dimension
cv2.waitKey(0)
cv2.destroyAllWindows()

**Main **

path = "./im3.jpg"

lz77Compress (path,500,500)

RGB image has 3 channels (in most cases). Try adding ch = img.shape[2] after col = img.shape[1] and tot = row * col * ch. Or better: tot = img.size. In case it's not the problem, please post a code sample that reproduces the problem - a code that someone can copy, paste and execute. — Rotem
i added ch but now I have this probelm : File "c:/Users/anton/Desktop/Progetto/test.py", line 17, in <module> lz77Compress (path,500,500) File "c:\Users\anton\Desktop\Progetto\image.py", line 35, in lz77Compress encodeCharacters = flat[selChar] IndexError: index 206 is out of bounds for axis 0 with size 206 — giangri_93
I case you decide to post a code that I can actually execute, please let me know. — Rotem
@Rotem in the comment i can't poste the code because is long, i give you my link github github.com/Anto-9393/project-multimedia-lz77. Thanks for your Help — giangri_93

Rotem Rotem · Accepted Answer · 2021-04-03T22:09:01

In most cases, RGB image has 3 color channels, and gray image has 1 color channel.

Assume img is a NumPy array that represents a gray image:
img.shape is going to be (rows, cols), and img.size is rows*cols.
Assume img is a NumPy array that represents a RGB (or BGR) image:
img.shape is going to be (rows, cols, ch), and img.size is rows*cols*ch.
In case of RGB, ch = 3.

I downloaded your code from github.
I saw you already corrected the code to support RGB images.

I can't reproduce the error in line 35 of image.py
Did you fix it?

The issues I could find:

You are not closing the files:
Close the files at the end of lz77Compress:
```
 # We must close the files.
 output.close()
 imgSize.close()
```

You better also close imsize after reading row, col, ch in lz77Decompressor.

When using cv2.imshow("output", decodeArray), you need the type of decodeArray to be uint8.
You may use the following code to show the output:
```
 cv2.imshow("output", decodeArray.astype(np.uint8)) # Show as uint8
```

In case you want to support both gray and RGB, you can check if len(img.shape) is 2 or 3, and add some logic at the Decompressor.

Note:
I didn't review the entire code, so there might be problems that I missed.

I am implementing lz77 compression for images. But if I insert an RGB image the output is wrong could any of you help me?

1 Answers