JPEG

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2008-03-05 USPTO Determination Granting Reexam of 5,253,341 C1

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JPEG (Joint Photographic Experts Group) is a commonly used method of lossy compression for digital images, particularly for those images produced by digital photography. The degree of compression can be adjusted, allowing a selectable tradeoff between storage size and image quality. JPEG typically achieves 10:1 compression with little perceptible loss in image quality.

Overview

JPEG compression is used in a number of image file formats. JPEG/Exif is the most common image format used by digital cameras and other photographic image capture devices; along with JPEG/JFIF, it is the most common format for storing and transmitting photographic images on the World Wide Web. These format variations are often not distinguished, and are simply called JPEG.

The JPEG standard includes a lossless coding mode, but in practice, JPEG is mostly used for lossy compression. The compression method is usually Discrete Cosine Transform (DCT), a type of Fourier transform. Color images are typically broken down into three color components (Red, Green, and Blue), and each component is then compressed separately. JPEG uses a chroma subsampling system, which reduces the resolution of the chroma components (color information) compared to the luma (light intensity) component, on the assumption that the human eye is less sensitive to fine color details than to fine brightness details.

Compression Process

The JPEG compression process involves several steps: 1. Color Space Conversion: The image is converted from RGB to a different color space, usually YCbCr, which separates the image into luma and chroma components. 2. Subsampling: The chroma components are subsampled to reduce the amount of color information, based on the assumption that the human eye is less sensitive to color variations than to brightness. 3. Discrete Cosine Transform (DCT): The image (or image blocks) is transformed from the spatial domain to the frequency domain using DCT. 4. Quantization: The DCT coefficients are quantized, reducing the precision of the high-frequency components more than the low-frequency components. 5. Entropy Coding: The quantized coefficients are then encoded using a form of lossless compression, typically Huffman coding.

File Formats

The most common JPEG file formats are: - JPEG/Exif, primarily used in digital cameras and other photographic equipment. - JPEG/JFIF, commonly used for storing and transmitting images on the web.

Quality Considerations

The main advantage of JPEG is its flexibility in compressing photographic images into a small file size. However, this compression can cause artifacts, such as blurring, blocking, and ringing around edges, especially at high compression levels. The quality of a JPEG image is directly related to its compression level; higher compression levels (resulting in smaller file sizes) will generally lead to lower image quality, while lower compression levels (larger file sizes) preserve more detail and produce higher quality images.

Applications and Usage

JPEG is widely used in digital photography, web design, and many other fields requiring efficient storage and transmission of photographic images. Despite the rise of newer formats like WebP and HEIF that offer better compression and quality characteristics, JPEG remains ubiquitous due to its wide compatibility and the vast amount of content already stored in this format.

See Also

• Digital Photography
• Image Compression
• Lossy Compression
• File Format

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