14 votes
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$8 \times 8$ block matrix in JPEG image compression?

The lossy JPEG compression does not merely remove small coefficients in higher frequencies. It encodes them with a precision relative to a (relatively crude) visual perception model; most notably, ...
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13 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 \...
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  • 5,780
10 votes

What is the least JPG-compressible pattern? (camera shooting piece of cloth, scale/angle/lighting may vary)

The composition below shows a fractal kind structure of the pattern. The every next picture is the result of averaging over each 2x2 pixels block of the previous one. The total character of the ...
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  • 141
10 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 ...
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  • 41.3k
8 votes
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Why Wavelet based Transform Is More Suitable for Image Compression Compared to DCT?

Both JPEG and JPEG 2000 use the change of basis compression type. Namely, we transform the data into a different representation assuming in this representation the number of parameters needed to ...
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  • 41.3k
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 ...
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  • 34k
7 votes
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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) ...
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  • 41.3k
6 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 ...
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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,...
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5 votes

How does a wavelet help in image compression?

A common wavelet based standard is JPEG 2000 and a common DCT based standard is JPEG. JPEG 2000 uses wavelets, but a good portion of the better compression it achieves than JPEG is due to the fact ...
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  • 1,071
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 ...
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  • 4,115
5 votes
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Encoding YUV Color Space with Positive Numbers

Why don't you use YUV with a shift? To make my example simpler, let's use the Color Space YCoCg. So the channels Co, Cg are in the range ...
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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. ...
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4 votes

Selective image compression?

To insert a JPEG image into another without recompression, you can use a jpegtran version with crop 'n' drop. It is safest to insert into coordinates that are multiples of the 16x16 pixel JPEG block ...
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4 votes
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Compare between JPEG and JPEG2000

JPEG is far simpler. It divides the image into 8x8 pixel blocks, and processes each using a Discrete Cosine Transform. The results are quantised and then encoded. The quality is fixed by the ...
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  • 517
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. ...
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  • 5,577
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,...
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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 ...
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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: "...
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4 votes
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How Can JPEG Be Used to Realize a Transfer Application with a Fixed Bit Rate?

You calculated the needed compression ratio correctly. Now to answer the rest of the question: The JPEG Algorithm doesn't let you set the size of the output file only the Quality measure. Namely to ...
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  • 41.3k
4 votes
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How Does Mean Centering Affect the Result of Using SVD to Compress Images?

I will try to illustrate why it is important to remove the Mean from data when doing the PCA (SVD is the tool to basically do the PCA approach to dimensionality reduction -> Compression). If the data ...
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  • 41.3k
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 ...
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3 votes

Image compression using SVD in MATLAB

The typical thing to do is the low-rank approximation on separate channels. Assume that $C$ is a channel of the RGB image $I$: ...
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  • 5,225
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 ...
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  • 26.8k
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 ...
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  • 26.8k
3 votes
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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\...
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3 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 ...
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3 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)...
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  • 26.8k
3 votes
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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 ...
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  • 26.8k
3 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 ...
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