All Questions
12 questions
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Discrete cosine transform and histogram equalisation
I found an image hash library for python and this blog
In the block Getting Funky With pHash, the author writes:
Further reduce the DCT. This is the magic step. Set the 64 hash bits
to 0 or 1 ...
2
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3
answers
567
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jpeg compression: necessity of dividing input image into 8x8 blocks
For jpeg compression, is it necessary to divide the input image into 8x8 blocks and perform a DCT on each of the 64 blocks?
If your input image is mxn, could you just have 1 mxn DCT coefficient matrix?...
- 21
0
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0
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59
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Intuition for feeding output from 'row' DCT in to 'column' DCT for JPEG
This question is motivated by 23:00 in this video.
It shows how the output from 'row' DCT is fed in to 'column' DCT for JPEG, and shows this image (highlights are mine):
In the top 'Rows' row my ...
- 101
2
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2
answers
2k
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Understanding gain of the DCT transform
I have been reading the guide from:
JPEG Huffman Coding Tutorial
I eventually reached the point where it states:
Finally, we want to convert the DCT DC value to an RGB value. Assuming
that the gain ...
- 164
1
vote
2
answers
1k
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DC Coefficient of JPEG Block
Suppose I take a grayscale image and break it into 8x8 pixel blocks, then take the DCT of those blocks. What is the DC coefficient of the block in relation to the whole image?
To simplify I'm ...
- 13
3
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2
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2k
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DCT - Coefficients and Basis Function
I am trying to understand how the Discrete Cosine Transformation works but I am not sure if I am at the right road.
Assuming that I have an $8\times8$ pixels image and I am applying the DCT to this ...
- 51
3
votes
1
answer
621
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What are the cosine functions in JPEG's DCT-II table?
As I understand it from this video, in a JPEG image an 8x8-pixel block is made up of weighted cosine waves, calculated using DCT-II. There are lots of visualizations of these waves, such as this one ...
- 133
1
vote
2
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103
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number of cosine waves in DCT
Why is it $N\cdot N$ a sufficient number of cosine waves equal to the size of the image or to a portion of it to describe it (or a portion of it) completely without losing information applying the DCT....
- 13
2
votes
1
answer
1k
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Image not divisable by 8? (JPEG Compression)
I have learned that an image is to be divided by 8*8 blocks, and the 2D DCT is then applied to these blocks. My question is, what if the image is not divisible by 8? What should I do in this case?
- 109
0
votes
1
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279
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How to undo the quantization step in DCT?
How can I do point by point multiplication of the quantized DCT coefficients with the quantization matrix before computing the inverse DCT for each block? I want to modify the reconstruction algorithm ...
- 125
3
votes
1
answer
398
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Determining the resulting pixel value before creating a compressed JPEG image
Is it possible to determine the resulting pixel values and modify them before the compressed jpeg image is actually created?
For example, I'm at the stage where I already have the quantized DCT ...
7
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2
answers
440
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JPEG steganography
Aside from Jsteg of Derek Upham, Outguess of Neils Provos, and F5 (improvement of F3 and F4) of Pfitzmann and Westfeld, What other methods can you use in information hiding in JPEG images?
The ...
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