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How does the JPG image format work?

The JPG (also known as JPEG) image format is one of the most widely used image formats on the internet. It is a lossy compression format, which means that some data is discarded in order to reduce the file size. This can result in a decrease in image quality, but the compression also makes it easier to store and transmit images.

The JPG format was first introduced in 1992 by the Joint Photographic Experts Group, hence the name JPG. It was designed specifically for compressing photographic images, and it is optimized for images with a lot of color and detail. The compression process works by analyzing the image and removing certain data that is deemed unnecessary. The amount of data that is removed depends on the compression level that is selected (named "Quality" in our image resizer).

One of the key features of the JPG format is that it is able to compress images to a relatively small size without sacrificing too much quality. This is achieved through a process called chroma subsampling, which reduces the amount of color information in an image. The human eye is less sensitive to changes in color than it is to changes in brightness, so chroma subsampling is able to remove some color information without significantly affecting the overall image quality.

The JPG format is also able to achieve a high level of compression by taking advantage of the fact that neighboring pixels in an image are often similar in color. It uses a process called discrete cosine transform (DCT) to analyze blocks of pixels in an image and remove the high-frequency components that contribute less to the overall image quality. The result is a more compact representation of the image data, which can be further compressed using a variety of algorithms.

One of the downsides of the JPG format is that it is a lossy compression format, which means that some data is discarded in order to achieve a smaller file size. This can result in a decrease in image quality, particularly if the compression level is set too high. Once an image has been compressed in the JPG format, it is not possible to recover the original data that was discarded.

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