CamCyclopedia
Image Compression Technology
File compression technology is a method of reducing the file size. Image and video data are generally large, so they are compressed to facilitate efficient storage and transmission. There are two types of compression methods; Lossy or irreversible compression, where size is reduced while preserving the original data as much as possible, and Lossless or reversible compression, where the size is reduced without changing the original data.
Below is a quality comparison of image compression in JPEG format. We can observe that when the compression ratio is higher, there is more quality degradation, which is visible as we move toward the left side of the image. [1]
What is lossy compression?
Lossy compression is a method of reducing the size of a file by changing the original file that the changes made do not substantially affect the quality of the file. The process of lossy compression preserves the important characteristics of the image data while incorporating the operating mechanism of the human eye so that the degradation involved is unnoticeable.
JPEG[2] is a representative example of lossy compression. The following JPEG block diagram demonstrates the flow of lossy compression.
Lossy compression takes advantage of the fact that the human eye is better at perceiving changes in brightness as opposed to changes in colour. Taking this into account, the lossy method reduces the size of the data by down-sampling the colour information after converting the file to YCbCr colour format, hence separating brightness and colour. What this does is that it retains the brightness of the image, but reduces the amount of colour information.
The lossy compression method considers another essential characteristic of the human eye to reduce the data size. The human eye is more sensitive to low-frequency changes in brightness rather than high-frequency changes. Thus, the lossy compression method discards high-frequency information which is less noticeable to the human eye.
Next, the compression ratio is increased according to the frequency of occurrence of data using Huffman coding. In simple words, this means that repeated elements are encoded together to decrease the amount of space they take up.
※ In contrast to lossy compression, the lossless compression method reduces the size of data by using the frequency of occurrence of data, without eliminating any information from the original data.
DNG can be saved in Samsung phones as shown in the following screenshots.
What is DNG?
DNG (Digital Negative) is an open-type, lossless raw image format developed by Adobe for digital photography [3]. The raw image file is an image file format containing data minimally processed from the image sensor of the digital camera or the image scanner [4]. Simply put, it is image data with all the details and information captured by the photographer, in its original and unedited form.
DNG uses the Raw file in its pristine form with lossless compression. This file is then formatted and used for processing or editing photos using editing tools such as Lightroom and Photoshop.
The following picture is an example of a post-edited DNG file.
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Notes
[1] Wikipedia https://ko.wikipedia.org/wiki/JPEG
[2] JPEG Standard (JPEG ISO/IEC 10918-1 ITU-T Recommendation T.81) at W3.org
[3] Wikipedia https://en.wikipedia.org/wiki/Digital_Negative
[4] Wikipedia https://en.wikipedia.org/wiki/Raw_image_format
※We will continue to add video compression format, MPEG used in Samsung phones and HEIF (High-Efficiency Image Format), etc.
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