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What Is Image Compression?

Updated on February 10, 2023
What Is Image Compression?

Welcome to the world of image compression! In today's digital age, images play a crucial role in our daily lives, from personal photos to professional graphics. However, images can also be quite large in size, leading to numerous problems, such as slow website loading times, limited storage space, and difficulties in quickly sharing images over the internet. This is where image compression comes into play, and in this article, we'll explore what it is, how it works, and what methods are available to help you come to a decision regarding which compression method is best suited for your particular use case.

What is image compression?

First and foremost, let's define what image compression is. Let's look at the definition.

Image compression is an application of data compression that encodes the original image with few bits. The objective of image compression is to reduce the redundancy of the image and to store or transmit data in an efficient form.

- An Introduction To Image Compression

In simple terms, image compression is the process of reducing the size of a digital image while retaining as much quality as possible. The goal of image compression is to make it easier to store, transmit, and display images, without sacrificing the image's visual quality.

At its core, image compression is when you remove or group together certain parts of an image file in order to reduce its size. Why do that? Here are a few reasons.

  • For website optimization. Sites with uncompressed images can take longer to load, and can cause your visitors to bounce because of this.
  • For reducing the amount of data being transmitted.
  • For sending and uploading images. Uploading an uncompressed image can take a while, and some email servers have a file size limit.
  • For reducing the storage impact on your hard drive.

How does image compression work?

There are two main types of image compression - lossless vs lossy. Let's take a quick look at them both.

Lossless compression

Lossless compression is a method used to reduce the size of a file while maintaining the same quality as before it was compressed. For example, in a DSLR camera, you probably have the option to save photos as either RAW or JPEG. RAW files have no compression and are great if you're a professional photo editor. But they take up more space. JPEG, on the other hand, won't fill up your hard drive as fast, but some of the data is lost in the conversion.

Types of lossless images include:

  • RAW - Found in many DSLRs, and keeps all the light data received from the camera's sensor. For a professional, this great news. However, these files types tend to be quite large in size. Additionally, there are different versions of RAW, and you may need certain software to edit the files.
  • PNG - Compresses images to keep their small size by looking for patterns on a photo, and compressing them together. The compression is reversible, so once you open a PNG file, the image recovers exactly.
  • BMP - A format found exclusively to Microsoft. It's lossless, but not frequently used.

It should also be noted that converting a lossy photo back to lossless won't restore the photo's data.

Lossy compression

In order to give the photo an even smaller size, lossy compression discards some parts of a photo. However, this doesn't mean the photo will look bad. Here are the two main types of lossy compression.


Also known as JPEG, this format gets rid of bits and pieces of a photo that you may notice depending upon the level of compression you apply. A normal amount of compression will not be noticeable, while extreme compression may be obvious.

There are also other ways a JPG image's quality may be reduced. If you rotate the JPG too much, you'll notice a difference in quality. This is because the photo has to recompress itself with every rotation, losing some data in the process. There are however programs out there that rotate a JPG losslessly. The same degradation applies if you save a JPG multiple times.


GIF compresses files by reducing the number of colors it has. If the photo has more than 256 colors (the maximum amount of colors older computers could have) this format will make the image look less appealing. The best use for GIFs are for images that are animated.

The example below shows a comparison between GIF images which range from 8 colors to 256 colors.

Methods of compression

Now that we've discussed various image formats, the following explains a few image compression methods used to achieve either lossless or lossy compression. These algorithms, or variations of these algorithms, are also what is used in image compression tools and services.


The Deflate method is a popular lossless image compression algorithm that uses a combination of the LZ77 compression algorithm and Huffman coding. The Deflate method is widely used in image compression and is the default compression method used by the PNG image format.

The LZ77 compression algorithm works by identifying and replacing repeated patterns in the image data with references to a single instance of the pattern. This process effectively reduces the size of the image data by replacing repeated patterns with a smaller reference.

Huffman coding is used in the Deflate method to further reduce the size of the image data by assigning short codes to frequently occurring patterns and longer codes to less frequently occurring patterns. This results in a more efficient representation of the image data, reducing its size even further.

The Deflate method is particularly well-suited to images with repeating patterns and few color variations, such as graphics and illustrations. It is also a lossless compression algorithm, which means that the image can be restored to its original form without any loss of quality.


Run-length encoding is a lossless image compression technique that is used to reduce the size of an image by encoding sequences of repeated pixels. This method is particularly useful for images that contain large areas of solid colors or repeating patterns, such as graphics or illustrations. In the Run-Length method, the image is divided into a series of scanlines, each of which represents a row of pixels in the image. The method then identifies sequences of repeated pixels in each scanline and encodes them as a single value, along with a count of how many times the pixel value is repeated.

Say you have a picture of red and white stripes, and there are 12 white pixels and 12 red pixels. Normally, the data for it would be written as WWWWWWWWWWWWRRRRRRRRRRRR, with W representing the white pixel and R the red pixel. Run length would put the data as 12W and 12R. Much smaller and simpler while still keeping the data unaltered.

The Run-Length method is a simple and effective image compression technique, but it does have some limitations. It is not well-suited to images that contain complex patterns or high levels of color variations, as these types of images typically contain many different pixel values, making it difficult to identify sequences of repeated pixels.

Arithmetic coding

Arithmetic coding is a type of lossless image compression method that provides higher compression ratios compared to traditional methods such as Huffman coding or Run-Length encoding. In arithmetic coding, the image data is represented as a single number, called a code, instead of a sequence of symbols. This code is a fraction between 0 and 1, and it represents the probability of the image data being a certain sequence of pixels.

In the arithmetic coding process, the image data is analyzed and a probability model is constructed based on the frequency of the different pixel values. The image data is then divided into a sequence of symbols, and the probabilities of each symbol are used to generate a code. The resulting code is then encoded into a binary representation that can be stored or transmitted more efficiently.

One of the key benefits of arithmetic coding is its ability to produce very small codes, even for highly complex images. This is because it takes into account the correlations between the different symbols, allowing it to encode the image data in a more compact form.

However, the arithmetic coding method can be computationally intensive and requires a large amount of memory, making it less suitable for real-time image compression. It is also less widely used compared to other image compression methods, due to the complexity of its implementation.


The Transform method of image compression is a lossless or lossy image compression technique that uses mathematical transformations to reduce the size of an image and commonly used for JPEGs. The idea behind the Transform method is to convert the image data into a different representation that is more compact, making it easier to compress.

One of the most widely used Transform methods is the Discrete Cosine Transform (DCT). In this method, the image is divided into small blocks of pixels and the pixel values in each block are transformed into a set of coefficients using a DCT formula. The DCT coefficients represent the relative importance of different frequencies in the image, and by discarding the lower frequency coefficients, which are typically less important for visual quality, the image can be compressed without a significant loss of quality. This is the basis for the JPEG image compression format, which uses a variant of the DCT method.

Another Transform method used in image compression is the Wavelet Transform. This method works by dividing the image data into a set of coefficients at different levels of resolution, and then only keeping the most significant coefficients. The result is an image representation that is more compact and easier to compress. The Wavelet Transform method is particularly well-suited to images that contain complex patterns or high levels of detail, as it can capture the important information in the image while discarding the less important details.

The Transform method of image compression is a powerful technique that can be used to reduce the size of an image while retaining its quality. However, the choice of Transform method and the specific parameters used can have a significant impact on the quality of the compressed image, so it is important to carefully consider these factors when choosing a Transform method for image compression.


Hopefully, this post helps answer the question "what is image compression?" and "how does image compression work?". As you can see, there are many moving parts. No matter how fast the Internet gets or how performant storage becomes, there will always be room for compression. It's important to note that image compression algorithms are not one-size-fits-all, and different algorithms are suited to different types of images. For example, some algorithms are better suited to images with few colors and simple patterns, while others are better suited to images with more complex patterns and color variations.

So, why is image compression so important in our digital lives? As mentioned earlier, images can be quite large in size, making it difficult to store, transmit, and display them efficiently. By compressing images, we can reduce their size, making it easier to store and transmit them and also improving website loading times. This is especially important for websites and applications that rely heavily on images, as faster loading times can improve the user experience and reduce bounce rates.

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