Tagged Questions

The Discrete Cosine Transform expresses a sequence of finitely many data points in terms of a sum of cosine functions oscillating at different frequencies.

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1answer
25 views

Why doesn't JPEG use 1D DCT for image compression?

I know that JPEG uses 2D dct and splits the images in 8x8 blocks. Why doesn't it simply split the image in one-dimensional vectors in $\mathbb{R}^{64}$? Wouldn't it simplify the math? My guess is that ...
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0answers
20 views

DCT Quantization Dark Image

I am trying to quantize a image after applying dct to a raw image.. But i am getting a dark image as the output.. What am i doing wrong.... I thought that adding a constant value across the whole ...
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2answers
28 views

How does using DCT help in image compression

I once learnt that the Huffman Encoding is used to rid of redundant information, thus reducing file size. I am not sure how using a DCT, or FFT on an image can help in reducing file size. It does not ...
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1answer
22 views

Do DCT each output coefficients have to be orthogonal?

I have a question: Do DCT each output coefficients have to be orthogonal? Here DCT is DCT-II type. It is used by the jpeg compression algorithm.
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46 views

Ads detection using MFCC and DTW

I am doing a project to detect Advertisements from transmission by using clipped segment (slogan) of an advertisement, based on looking at the audio track as follows: Audio Signal $\rightarrow$ ...
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1answer
75 views

Why zig zag manner scan is used in DCT for image compression?

I am not sure why zig zag manner scan is used in DCT. I can guess that might because it moves from low frequency component to high frequency component and as most of the energy is stored in low ...
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0answers
25 views

FFTW: Logical array size in r2r transform?

I'm taking a multidimensional discrete cosine transform (FFTW_REDFT11) in FFTW, and am unsure how to compute the scaling factor. According to the documentation, ...
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1answer
71 views

High energy coefficients in 1d DCT for audio

I've been reading a little bit about how DCT works in image compression using the 2D transform, however I don't know how this applies to audio using a 1D transform. Suppose I have an audio segment, I ...
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3answers
185 views

Is this a correct interpretation of the DCT step in MFCC calculation?

This is a continuation of the discussion here. I would comment there, but I don't have 50 rep so I'm asking a new question. Here's how I understand the DCT step in the MFCC calculation process: The ...
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2answers
134 views

Why would you use DCT as oppose to DFT for detecting oscillation in a signal?

For my project, I needed to detect the primary oscillation frequency in a signal data. As I googled around, it turns out that DCT is the best tool to find this ...
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1answer
43 views

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 ...
0
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1answer
56 views

How is compression ratio related to number of dct coefficients

Lets say we have a picture with 6bits/pixel and we use discrete cosine transform with 8x8 blocks of pixels and each coefficient is 8 bits. We can use 2 8x8 masks, a zonal mask A and a zonal bit ...
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1answer
72 views

NxM DCT matrix generation

How can I generate an NxM DCT matrix? In Matlab dctmtx function generates only square matrix (NxN).
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1answer
39 views

Is there a way to evaluate block size when doing image processing or video processing

I am currently doing foreground segmentation in image sequences. A DCT 8 by 8 feature is extracted for each pixel. Now i want to explore dynamic block size for different scenes. Is there any rules ...
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1answer
48 views

Why do we quantize high frequencies coarsely, shouldn't it be the opposite?

Also, in Huffman Coding, why do we use fewer bits for high frequency data. I don't get the intuition. Shouldn't it be the opposite or does this have to do something with the psychoacoustic model of ...
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3answers
180 views

What does the negative sign mean in the image frequency domain?

When I transform an image from the spatial domain to the frequency domain, some frequencies are negative. What is the meaning of the negative sign? For example:
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2answers
158 views

DFT Vs DCT - spectrum differences

When you are transforming a signal in to a frequency domain using both DFT and DCT, say for a function sin(x), how the spectrum will be? will both frequency values (of DCT and DFT) same? Can anyone ...
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0answers
83 views

On application of MFCC for sound classification. How to normalize MFCCs

I have found some explanations on MFCC and specifically on application of DCT. I feel like I am slowly getting to the point but I still find a lot of things that confuse me. Below is the part of the ...
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0answers
46 views

Inverse Cosine Transform via FFT

I've read a post and implemented a code that runs DCT via FFT and padding. Fast Cosine Transform via FFT I am looking for an idea of how to implement IDCT or also called DCT type 3. I assume there ...
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2answers
162 views

DCT step suppress high frequencies in MFCC extraction process

I'm trying to get MFCC from speech audio data. I'm using 26 Mel Filter on 0-24kHz spectrum. My MFCCs doesn't look correct to me, cause always first bins have high values and values getting smaller ...
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2answers
466 views

Conversion between DCT to DFT frequency domains

It is well known that the DFT can be interpreted as a filter bank. The same can be done for the DCT, for instance Fig. 3.29, pag. 129 in [1] shows a plot of the frequency response of a DCT filter ...
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4answers
465 views

Why are there only a few high frequency components in the Discrete Cosine Transform?

I am learning about the Discrete Cosine Transform (DCT) and I read somewhere that, in general, images have very few high frequency components. Can someone give me an intuitive explanation for why this ...
2
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1answer
294 views

DCT of 4x4 checkerboard matrix - what's the correct result?

I'm trying to work out what the correct solution is for a DCT of a 4x4 checkerboard matrix (let's call it A) and for a matrix of ones (let's call it B). So A is: ...
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1answer
365 views

Can the Walsh Hadamard transform be computed for non power of 2 sizes?

Can the walsh hadamard transform be calculated for odd image block sizes such as 5x5 or 7x7? Most of the examples I've seen are for 4x4 and 8x8? I fear it probably can't from the description I read ...
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1answer
755 views

Relationship between DCT and PCA

I have a basic implementation knowledge of the 2D 8x8 DCT used in image & video compression. Whilst reading about Principle Component Analysis, I can see a lot of similarity, albeit PCA is clearly ...
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votes
1answer
257 views

DCT and hard thresholding

If I have an Image and i find the DCT and then apply hard thresholding on the coefficients and then IDCT then I have attenuated the noise. Can someone please explain in detail or point me to the ...
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2answers
4k views

Fast Cosine Transform via FFT

I want to implement the Fast Cosine Transform. I read on wikipedia, that there is a fast version of the DCT which is similarly computed to the FFT. I tried to read the cited Makhoul* paper, for the ...
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2answers
162 views

Why motion cannot be estimated in DCT domain using block match?

In all video coding standards, motion estimation is always done in pixel domain using block matching. I wonder why it cannot be estimated in DCT(Discrete cosine transform) domain using block matching ...
3
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1answer
254 views

DCT Steganograhy

I'm trying to implement a simple steganography algorithm (similar to JSteg algorithm) for hiding data in LSB of DCT coefficients in Matlab, and I ran into a problem. a. I apply DCT. b. modify LSB ...
3
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1answer
252 views

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 ...
6
votes
1answer
313 views

Non-orthogonal basis which offers better energy compaction in image compression

I am aware of image-independent basis, i.e. DCT, and image-dependent basis, i.e. Karhunen–Loève, which are used in compacting energies for image compression. These bases are orthorgonal. Are there ...
5
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2answers
240 views

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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4answers
1k views

Do the DCT coefficients correspond to particular pixels?

Getting the DCT coefficient based on what I've read, is usually done through a matrix, usually 8x8 or 16x16 using these formulas: ...
0
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1answer
367 views

1-D discrete cosine transform(DCT)

This was a past year paper question. Not sure how to answer it. Question: The 1-D discrete cosine transform(DCT) of a sequence f(x), x =0,1,...,N-1 is F(u) = c(u)* Summation of f(x) cos ( ...
9
votes
1answer
693 views

Could a DCT be used for an audio magnitude spectrum rather than DFT?

From what I understand, the DCT has half the bin size as a DFT of the same size N. The DFT also includes phase information, but often this is not needed when only the magnitude spectrum is desired. ...
34
votes
4answers
12k views

What is the difference between a Fourier transform and a cosine transform?

In speech recognition, the front end generally does signal processing to allow feature extraction from the audio stream. A discrete Fourier transform (DFT) is applied twice in this process. The first ...