JPEG Image Compression Standard

• JPEG (Joint Photographic Experts Group) is an image compression standard.
• It was formally accepted as an international standard in 1992.
• JPEG uses a transform coding method using the DCT (Discrete Cosine Transform).
• This method transforms the spatial domain into the frequency domain.

Progressive Image Compression

• Progressive compression is useful for transmitting compressed images over a communications line.
• Images are decompressed and viewed in real time.
• Upon receiving and decompressing an image, the decoder quickly displays the entire image in low quality.
• The display quality improves as more and more of the image is received and decompressed.
• The encoder compresses the most important image information first.
• It then compresses less important information and appends it to the compressed stream.
• Progressive image compression methods have a natural lossy option, that involve stopping compression at a certain point.
• JPEG uses the DCT to break an image into spatial frequency components for progressive image compression.
• It compresses the low-frequency components first, allowing the decoder to quickly display these parts.
• Low-frequency parts contain the principal image information.
• High-frequency parts contain image details.
• Thus, JPEG encodes spatial frequency data progressively.

JPEG Compression Modes

• The JPEG standard defines four compression modes: Hierarchical, Progressive, Sequential, and Lossless.
• Sequential-mode images are encoded from top to bottom.
• Sequential mode supports sample data with 8 and 12 bits of precision.
• Image components are compressed either individually or in groups (by interleaving).
• Sequential mode is a One pass operation.
• "Baseline System" is a restricted mode that must be included in any decoder.
• Color Components Interleaving is done to save buffer size.
• Progressive JPEG images components are encoded in multiple scans each code a part of the quantized DCT coefficients data.
• There are two ways for progressive images:

Spectral Selection

• Coefficients are grouped into spectral bands, and lower-frequency bands are sent first.
• This takes advantage of the "spectral" (spatial frequency spectrum) characteristics of the DCT coefficients.
• Higher AC components provide detail information.
• Scan 1 encodes DC and the first few AC components (e.g., AC1, AC2).
• Scan 2 encodes more AC components (e.g., AC3, AC4, AC5).
• Scan k encodes the last few ACs (e.g., AC61, AC62, AC63).

Successive Approximation

• Instead of encoding spectral bands gradually, all DCT coefficients are encoded simultaneously.
• This done with their most significant bits (MSBs) first.
• Scan 1 encodes the first few MSBs (e.g., Bits 7, 6, 5, 4).
• Scan 2 encodes less significant bits (e.g., Bit 3).
• Scan m encodes the least significant bit (LSB), Bit 0.
• The top-left corner entry with the rather large magnitude, also called the constant component, defines the basic hue for the entire block, as well as the alternating components.

Lossless Compression

• A type of data compression algorithms that allows original data to be perfectly reconstructed from compressed data

Hierarchical Mode

• It is a super-progressive mode in which an image is broken down into frames.
• A frame is a collection of one or more scans.
• The first frame creates a low-resolution version of the image.
• Remaining frames refine the image by increasing the solution.
• JPEG is a sophisticated lossy/lossless compression method for color or grayscale still images but does not handle bi-level (black and white).
• It works best on continuous-tone images, where adjacent pixels have similar colors.
• Hierarchical Mode is useful for multi-resolution requirements.