Questions tagged [jpeg]
JPEG is a lossy image compression format, suitable for photographs and other natural images. It attempts to compress the image while retaining its visual appearance, while allowing the compressor to trade image quality for file size.
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Why does MP3 use modified DCT?
I read on Wikipedia MP3 that the MP3 standard uses a modified discrete cosine transform.
Why does it use a modified transform and not the original DCT like the JPEG standard? Are there reasons ...
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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 ...
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Measurement of blocking artifacts in images
I want to define a threshold for maximal blocking artifacts in an image.
What is a good key value to measure blocking artifacts?
I have no reference image
non natural image
Example:
I looked into ...
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What happens when you read and save the same JPEG image again over and over?
This question is mostly related to jpeg compression and expected results when you read and save the same jpeg file several times.
Let's say you have read an uncompressed image (originally it has never ...
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Why doesn't JPEG file format follow indexed color technique?
This is table about "Image file formats supporting indexed color".
I don't see JPEG file format in this table.
Why doesn't JPEG file format follow indexed color technique?
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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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JPEG-LS: How to pick T1, T2 and T3 preset coding parameters?
I am reading ISO/IEC 14495-1 : 1999 (E). Under section C.2.4.1.1 JPEG-LS preset coding parameters, one can find the valid values ...
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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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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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Perfect lossless rotation of JPEG image (ITU 81)
JPEG image compression is Fourier based DCT (ITU 81). It divides the image into 8x8 pixel blocks, and processes each using a Discrete Cosine Transform. The results are quantised and then encoded.
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What is facebook's image compression algorithm?
For a project I'm working on (master thesis), I need to figure out what is facebook's image compression algorithm.
The goal: to be able to mimic facebook's image compression locally (using python).
...
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Does decoding jpeg images uncompresses them?
I train a convolutional neural network on compressed jpeg images. I noticed that my code takes pretty much the same amount of space in memory whether I use high quality jpeg images or the same jpeg ...
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How Does Color Conversion and Downsampling work When JPEG compressing a grayscale image?
For color images I know you have to convert to YCbCr format, and then for downsampling is based on the fact that human beings see differences in Y more than differences in Cb or Cr.
How do I do these ...
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Need 600dpi JPEG file smaller than 5MB
In order to submit a document to an online website, they require the following:
Scan the US letter-size hard-copy document at 600 dpi in greyscale
Provide a JPEG, JPG, or PNG file of the scan with a ...
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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 ...
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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 ...
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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 ...
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How do 2D DCT basis functions interact to produce 8x8 JPEG tiles?
This section is a bit of an intro. If you want to skip directly to where the problem is explicitly outlined, skip to the bottom of this post. Please note that I'm not hip with mathematical lingo, so ...
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Blocking effect after reconstructuring the image. Jpeg compression [closed]
The image is splitted into 8x8 blocks each of which are processed through DCT, quantization, huffman encoding/decoding, dequantization and inverse dct and used to recontruct the image. But there is a ...
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JPEG compression steps after quantization
I have a 3-channel (for colours) a png image that I opened and
I splitted the image into 8x8 blocks
I applied all of the blocks discrete cosine transform
And then applied quantization
I stored the ...
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Is this jumbled image due to difference between Matlab and Python slicing?
When I import an image from Matlab I get the following jumbled image
rather than the (224x224x3 RGB) image of a single elephant I expected
I write the image from Matlab as a binary file
...
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JPEG: In image compression why are higher frequency components divided by a larger number during quantization
Once 2D FDCT has been carried out on 8x8 blocks of the image data, the coefficients are quantized. This involves use of a quantization matrix where we simply divide each FDCT coefficient by the ...
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How Can JPEG Be Used to Realize a Transfer Application with a Fixed Bit Rate? [closed]
For simplicity, assume a fixed bandwidth (5MB/s) and that each second an image with 12MP is generated. The quality of the image ...
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Do JPEG formats add significant overhead?
For an extreme example, I have the following image that was a JPEG before stackexchange converted it to a PNG:-
Before you perform all sorts of funky image processing tricks on it, it's just a black ...
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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....
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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?
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How to remove artefacts from this image?
There is some vertical banding near the tree trunks in the following image.
Is there a way remove this type of artefact, preferably using Mathematica?
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What pattern maximises a JPEG file's size? [duplicate]
As I understand the JPEG algorithm, it smooths (ignores) gradual variations in colour and brightness in favour of high frequencies. This loss of information is what creates the characteristic high ...
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Confirming what a compression ratio is
I have looked at various sources, but still have not been able to answer the question: what is compression ratio? I have come to the understanding that this is the definitive answer:
compression ...
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compression ratio of pixels block
I am new to image compression topic and really appreciate it if someone could help me understand how compression rati works in this case. Assuming I have JPEG lossless coder that encodes the following ...
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YCbCr to jpeg-YCbCr
Im sampling an image in YCbCr from an ov7670 camera and using jpegant library to encode a jpeg file.
Whenever I do it directly using Y,Cb,Cr values from my camera, I get a pink-violet version of the ...
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How doesn't JPEG huffman table contain 256 codes?
Here are the DHT entries I got from JPEG snoop:
...
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How does Jpeg encoding affect the final file size of 24BPP RGB and 8BPP grayscale image?
The reason I'd like to know this is because of a file size problem. I am trying to reduce an image's file size from converting it from TIFF to JPEG. A user challenge me that he notice a RGB TIFF image ...
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Does every 8 by 8 DCT matrix have a valid (in range) IDCT?
For a given quantization level of JPEG compression, are all possible DCT matrices have valid IDCTs. By valid, I mean matrices whose elements are in the range [0,255] ?
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How many 8x8 JPEGs exist with distinct DCT given a quality level?
For a given quality of image (or quality of compression), Multiple 8x8 JPEG images can have the same DCT (Discrete cosine transform) matrix, for a given quality of JPEG compression. I want to find the ...
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2D DWT computation order
In 2D the discrete wavelet transform (DWT) of an image using lifting based 5/3 filter, if I perform a row-wise operation first then perform column-wise operation then I will get 4 sub-band LL, LH and ...
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Which image filter can be applied to remove gridded pattern from corrupt jpegs?
I have around 1,400 jpegs that have been corrupted somehow and have lost the backup images. They all seem to have the same gridded pattern of lines over each (i.e. the gridding does not shift from ...
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In a JPEG file, how much is noise and how much is the image?
I have an IP webcam that generates very low light images of a plain background. The camera spits out JPEG only files. The following is one such frame...
and this is with graphic equalisation ...
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JPEG DCT padding
Since the JPEG DCT block used is 8x8, how does the method deal with images with dimensions that are not multiples of 8? What kind of padding does it use? How are the 8x8 blocks of the image chosen?
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What base image size to choose to assist easier downsizing?
Android applications need image at different resolutions to support different screen densities and pixel size.
I am planning to provide only one image with high pixel size and let the underlying ...
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Why are some JPEG files of the same scene much larger than others?
I'm using a Foscam FI8910W ip camera to view a static scene under constant lighting conditions. When I pull back a frame grab, it's about 35 KB in size. I can do this over and over and it's always ...
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What would produce this "hamburger" corruption on images?
I have an image that has gone through an unknown number of jpeg and webp conversions (at least one but more likely 2+). Currently this image is a JPEG and contains corruptions in single color or mild ...
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Performing image equalisation and then Jpeg compression
I'm trying to estimate what would be the result of first performing an histogram equalisation on an image, and then compressing it using JPEG.
I know that histogram equalisation increases the ...
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Difference between jpg compression and mp3 compression
My understanding of jpg compression is that, if done repeatedly, any image will eventually be reduced to a single flat color, due to the way it deals with neighboring pixels. However, years ago I was ...
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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 ...
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What is the Compression Algorithm used by Facebook?
I would like to know what is the Compression Algorithm used by Facebook for compressing the images? I uploaded a image of size 242 Kb and I downloaded the same image from my Facebook album. The size ...
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Usage of Digital Cosine Transform in lossy compression of images
I was reading about Digital Cosine Transform (DCT Type-I) and it's use in JPEG.
I have the following doubt. My teacher offered an explanation which wasn't very convincing.
What property of Digital ...
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What is the least JPG-compressible pattern? (camera shooting piece of cloth, scale/angle/lighting may vary)
I am trying to design a cloth that, from the point of view of a camera, is very difficult to compress with JPG, resulting in big-size files (or leading to low image quality if file size is fixed).
It ...
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Retrieve pixel values from the output jpeg image
I have this jpeg image (abc.jpg) and I change some pixel values of this image and store this on hard disk, to later see the changed the pixel values. The name of the output jpeg image is say, xyz.jpg. ...
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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 ...
