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15 votes
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Does a simple photograph contain more information than a complex painting?

It depends how you define the term "information" or "entropy". The conventional definition of entropy of an image is to think the image as a two-dimensional matrix of pixels and $$H = - \sum_k p_k \...
AlexTP's user avatar
  • 6,595
10 votes
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Pros, Cons, and Applications of SVD-based Image Compression (Singular Value Decomposition)

The idea of using PCA with images is to look at the margin ensembles generated by the image. That means, we consider two distributions: The distribution of the column over the realisations of the rows....
Jazzmaniac's user avatar
  • 4,583
8 votes
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Loss in converting YUV to PNG

The PNG format is lossless for RGB24 image data. However the conversion from YUV to RGB24 is not lossless, as the two formats quantize the color space differently. To see this, the following ...
hotpaw2's user avatar
  • 35.4k
7 votes
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Why choose anything but highest compression for PNG?

Simply because the highest compression typically is significantly more CPU-intense (it tries out multiple different approaches to represent successive lines). This really shouldn't make much ...
Marcus Müller's user avatar
6 votes
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How does DCT decorrelate images?

[EDIT] In 1991, Nasir Ahmed wrote: "How I Came Up with the Discrete Cosine Transform". Interesting to read, on how he was inspired by Chebyshev polynomials, and on how he didn't get funding,...
Laurent Duval's user avatar
6 votes
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Universal Bases (Dictionary) for Image Compression

This is a great and interesting question. There are 2 ways to look at it, empirically and analytically. But before we start, a major detail is that when dealing with images we mainly talk about the ...
Royi's user avatar
  • 19.7k
5 votes

What are some good images to use when comparing image compression algorithms?

Are there images that are better suited to testing compression quality than others? You are exactly right. A good compression algorithm is one that performs well on average considering all those ...
msm's user avatar
  • 4,295
5 votes

Universal Bases (Dictionary) for Image Compression

As a complement to the neat answer by @Royi, I would add that "sparsity" is originally a heuristic principle in science, that applies well to many interesting really world data and problems. ...
Laurent Duval's user avatar
4 votes

Does every 8 by 8 DCT matrix have a valid (in range) IDCT?

All images blocks that are applied by DCT matrix have a valid IDCT - i.e. they can always bring back original pixels and in general inverse transfer is theoretically as well as computationally viable. ...
Dipan Mehta's user avatar
  • 5,597
4 votes
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JPEG DCT padding

The filling is performed to the right ($[1\,,1\,,3\,,x_1\,,x_2\,,x_3\,,x_4\,,x_5]$) or the bottom ($[1\,,1\,,3\,,y_1\,,y_2\,,y_3\,,y_4\,,y_5]^T$), line by line or column by column. The extended values,...
Laurent Duval's user avatar
4 votes

Are the FFT coefficients symmetric in image processing?

For real images, there is indeed a formal redundancy, termed Hermitian or conjugate-symmetric as detailed by @Fat32. This symmetry however is "modulated" by the complex expression of the Fourier ...
Laurent Duval's user avatar
4 votes
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Confirming what a compression ratio is

Neither. A true compression ratio is: "original file size in bits" divided by "compressed file size in bits". A practical (based on disk limits) compression may embed the chunk size effect: "...
Laurent Duval's user avatar
4 votes

what is the difference between YUV format and HSV format?

So the best discussion I can point to is in this PDF on Charles Poynton's site. The PDF goes on to much more detailed explanations of each color space, and how to convert between them. It's a bit ...
Peter K.'s user avatar
  • 25.8k
4 votes

what is the difference between YUV format and HSV format?

YUV is basically a member of the television broadcast industry video/image color encoding formats, and was created during the transition of black & white broadcast into color. A backwards (B&W)...
Fat32's user avatar
  • 28.3k
4 votes

Where is truncated-SVD image compression actually used?

A first difficulty with your question is that "usual instances" may not be fully publicized nor documented. A second one is related to the fact that "compression standard" are ...
Laurent Duval's user avatar
3 votes

compression ratio of pixels block

Compression ratio is $$ \frac{N_{uncomp}^b}{N_{comp}^b}$$ where ${N_{comp}^b}$ is the total number bits requried to represent those $3$ pixels $A$,$B$, and $C$ while ${N_{uncomp}^b}$ is the total ...
Fat32's user avatar
  • 28.3k
3 votes

Are the FFT coefficients symmetric in image processing?

Your observation is basically right. Given a 2D discrete-space real sequence $x[n_1,n_2]$, its 2D discrete-space Fourier transform $X(e^{j{\omega}_1},e^{j{\omega}_2})$ is a complex valued function ...
Fat32's user avatar
  • 28.3k
3 votes
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How does Jpeg encoding affect the final file size of 24BPP RGB and 8BPP grayscale image?

Take a standard R-G-B coded 24-bit image. First, suppose that all channels are coded independently. If by chance, R, G, and B are equal, each one will be converted to the same JPEG file with size $s$. ...
Laurent Duval's user avatar
3 votes

Random Noise Removal In Images

There are a lot of ways to do image denoising. I will give several known examples: Hard/soft thrsholding of the wavelet decomposition: Do a wavelet decomposition of the image and for zero all ...
Amitay Nachmani's user avatar
3 votes
Accepted

What are the cosine functions in JPEG's DCT-II table?

Those 2D cosine functions are independent of your input image. They are just "cosine waves", or the 64 basis functions that yield 64 coefficients when transforming "$8\times 8$" blocks. Given a $D$ $8\...
Laurent Duval's user avatar
3 votes
Accepted

Apply Principal Component Analysis (PCA) for RGB Images

General Idea The general idea of Principal Component Analysis (PCA) is as following (Intuition over formalism): Given a set of points in space (Inner Product Space) find a set of vectors (Directions) ...
Royi's user avatar
  • 19.7k
3 votes

Definition of difference between compression and filtering

My personal view: it depends a bit whom you talk to. In the SP community, if people talk about "filters", more than 90% of the time, they will refer to linear filtering, in particular filters that ...
Florian's user avatar
  • 2,463
3 votes
Accepted

Image compression

Thresholding alone cannot perform data compression. You need to employ some encoding strategy to discard those ignored coefficients, to achive actual bit reduction. For example, in lossy JPEG image ...
Fat32's user avatar
  • 28.3k
3 votes

How Does Color Conversion and Downsampling work When JPEG compressing a grayscale image?

Normally, monochrome images should be stored as images with a single channel. However, I have seen digital images with three channels containing the same values. This results in a displayed image ...
Laurent Duval's user avatar
3 votes

What happens when you read and save the same JPEG image again over and over?

JPEG is lossy compression, and it is allowed to do anything deemed beneficial in representing an image as accurately as possible using the minimum amount of storage while keeping cpu load in check. ...
Knut Inge's user avatar
  • 3,434
2 votes

Skip corrupt part of jpeg

Often it's possible to detect invalid markers or Huffman decompression errors. If bottom part is grey though, it's most likely invalid marker inside JPEG image data. JPEGSnoop will tell exact byte, ...
Joep van Steen's user avatar
2 votes

How does a wavelet help in image compression?

Continuous wavelets are indeed functions that wiggle and eventually vanish. For the discrete ones, used in image compression, their is another point of view. Given a block $B$ of pixels of size $2K\...
Laurent Duval's user avatar
2 votes

How does a wavelet help in image compression?

I think that modern video codecs using block transforms in intra mode have better subjective/objective compression or at least on par with jpeg 2000. See eg: https://hal.archives-ouvertes.fr/hal-...
Knut Inge's user avatar
  • 3,434
2 votes
Accepted

The magnitude of taking discrete cosine transform of an image two times is similar to the original

It's beacuse the inverse discrete fourier transform (DCT) is almost identical to the forward DCT. So taking twice the transform will be similar to the original signal. In fact if you provide which DCT ...
Fat32's user avatar
  • 28.3k
2 votes

JPEG DCT padding

What if image dimension not a multiple of 8?How padding works? If the image width and height are not multiple of 8, the image will be padded with zeroes most commonly.. How 8*8 blocks are chosen ...
Navin Prashath's user avatar

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