# Questions tagged [dct]

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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### 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?...
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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 ...
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### Is Discrete cosine transform Translational Invariant

I am using 2D Mellin-Cepstrum to extract features, If I use log-polar transform to get rotational invariance, the rotation is transformed to translation. Can I use DCT next to get translational ...
82 views

### What frequency does each bin of a Discrete Cosine Transform represent?

I have applied DCT, type 2 to a vector of values from a sensor. For example it has length N = 8. I received a vector of DCT coefficients. How can i find out the frequency of each bin in Hz? The ...
218 views

### 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 ...
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### Inverse DCT4 by using Inverse DFT

Say I have an inverse DFT function ready for use. I am struggling to find online or mathematically deduce the additional steps needed to compute an inverse DCT4 by using said inverse DFT function. If ...
1 vote
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1 vote
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### Calculating DCT in reversed vector

I'm doing an exercise in which I need to show that the DCT of $\tilde{x} = (x_{N-1}, x_{N-2}, ..., x_1, x_0)$, with $\tilde x_m = x_{N-m-1}$, is equal to $\tilde{X}_k = (-1)^{k}X_{k}$, but I have ...
1 vote
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### On the performance of DCT-based OFDM

I am trying to understand the OFDM based on DCT instead of DFT in order to compare between their performance. In case of AWGN, the performance is logic, but when I perform channel equalization in DCT-...
181 views

### KLT vs DCT basis restriction error

I want to implement 8x8 DCT and 8x8 KLT matrices to the 8-length vectors of 256x256 Lena image (e.g. each row of image there is 32 vectors 256/8 = 32 and for all rows 256x32 = 8192). I do the eigen ...
718 views

### Convolution Theorem using DCT

I am trying to digest this paper.  Anna Usakova. Using Of Discrete Orthogonal Transforms For Convolution So far, I've been successful with DFT and DHT. Could someone give a hand with DCT? I am ...
209 views

### Can we use discrete cosine transform with imaginary part

I have been reading about the difference between the use of iFFT and iDCT with OFDM. What I understood that iDCT can only be used with one phase modulated symbol which is PAM/BPSK modulation. It ...
1 vote
944 views

### Discrete Cosine Transform (DCT) Coefficient Distribution

I have two images : Original Image Binarize Image I have applied Discrete Cosine Transform to the two images by dividing the 256x256 image into 8x8 blocks. After, I want to compare their DCT ...
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### Why do we use the term "cepstral" in MFCCs, and how does it related to the cepstral domain?

As I know, we can get the cepstrum by taking the Inverse Discrete Fourier Transform (IDFT) of the log of the power spectrum. And we can get the Mel-frequency cepstral coefficients (MFCCs) by taking ...
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### When to Apply Circular Convolution Formulas?

Context I am studying the family of Discrete Trignometric Transforms (DTT): Discrete Cosine Transforms (DCT) and Discrete Sine Transforms (DST). And trying to understanding more their properties, I ...
1 vote
631 views

### 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 ...
906 views

### 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 ...
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### DFT and integer valued basis functions'sf requencies

In Matlab the function W=dftmtx(N) gives the DFT matrix of size N. Each row is computed for an integer frequency k. $W_{k,n} = e^{-i2\pi kn/N}$, k-th frequency, ...
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### How to use a size N complex DFT to compute a size N/2 real DCT? [duplicate]

Can someone (maybe @hotpaw?) Explain How to use a size N complex DFT to compute a size N/2 real DCT? Also, I noticed that the DCT matrix (which can be obtained in Matlab by dctmtx(n)) contains dct ...
1 vote
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### DCT vs DFT why do we need/want phase?

Why do we use complex exponentials in the Fourier transform, why do we want the phase part? As opposed to in DCT where there is no phase and only magnitude? Moreover, what does this phase concretely ...
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### Why is the Fourier (or cosine) transform decorrelating?

The discrete Fourier transform (DFT) and the discrete cosine transform (DCT) both decompose a signal into its frequency-domain spectrum. One property that I have seen praised across various domains ...
277 views

### 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 ...
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### What frequency each bin of a Discrete Cosine Transform represents?

A practical example: I perform a DCT on a time series of discrete values that are spaced in time by 1/30 of a second. What frequencies each bin of this DCT represents? What is the formula to find that?...
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### How do I find the frequency of a particular bin in a cosine transform, and two other questions

I have a discrete series of values and I have applied a Discrete Cosine Transform to it, obtaining values like: ...
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### How to get a heartbeat signal from this data?

I am giving my first steps in data analysis, gathering/cleaning. To learn, I am trying to create a simple code that can detect heartbeats from color variations from the image coming from the camera ...
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### DCT Based modelling

I have been reading this paper on MRI and I only do have novice level signal processing insight. I could not understand the following model: The displacement field (i.e. d) is modelled as a linear ...
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### Do I need equally paced values to do a Fourier Cosine Transform?

I am gathering data from a sensor, but the data gathering depends on a code running, so the timing is not precise to make it equally paced. What I mean is this, the time it takes to do measurement #1 ...
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### How to keep low frequencies in DCT

How can I keep a percentage r of the low frequencies of a Nx1 signal x using the Discrete Cosine Transform of it? Is it simply zeroing all but the first rN elements of DCT(x)?
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### Is discrete cosine transform differentiable?

Is DCT differentiable? I.e. is $\frac{\mathrm{d}\mathrm{DCT}(x)}{\mathrm{d} x}$ defined? I've seen a few implementations in automatic differentiation software packages, so I suppose the answer is ...
1 vote
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### How to determine a quantization table for image compression with DCT?

The quantization table is given on the JPEG Wiki page. How was this table derived? I'm currently doing a project where I have to come up with custom quantization table for an image, but I don't know ...
2k views

### Computation of the Inverse DCT (IDCT) Using DCT Or IFFT

Is there a way to compute the inverse discrete cosine transform (type-2) by leveraging either a DCT, FFT, or IFFT algorithm? I have seen ways to compute the DCT using FFTs, and I've seen ways to ...
1 vote
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### What is wrong with my 3D DCT quantization?

I'm implementing 3D DCT quantization as in Quantization of 3D-DCT Coefficients and Scan Order for Video Compression (sorry about the paywall). They propose the following equation for generating the ...