PLAN: 2 Why compression ? About digital images. Lossless data compression. Lossy data compression. Conclusion.
Why compression ? 3 Image with 10 Megapixel Three color component (R,G,B). One byte by component. Memory occupation : 30 Mbyte by image. Stocking on disk ? Transmission over the network ? Reduce its size on disk. Speed transmission over a network.
About images 4It is acquired, created, processed and stored in binaryform. It is composed of a set of points called pixels.Example: Image 1024 × 768 coded on 3 bytesNumber of pixels : 1024 × 768 = 786432 pixelsImage size : 1024 × 768 × 3 = 2359296 octets = 2,25 Mo
Performance criteria 5 Compression ratio is an important factor to differ between images : The Mean Square Error is the cumulative squared errorbetween the compressed and the original image :
Algorithm compression 6There are two types of compression:Lossless compression : Perfect reconstruction. Statistical redundancy. Small compression ratio.Lossy compression : Reconstructed image ≠ original image. Quantization. Visually lossless. High compression ratio.
Lossless compression 7There are three types of losslesscompression:•Methods based redundancy (RLE).•Statistical methods (Huffman).•Methods based on dictionaries (LZW).
Run-length encoding 8 Definition : Run-length encoding is a data compression algorithm that is supported by most bitmap file formats, such as TIFF, BMP, and PCX. Principle: RLE works by reducing the physical size of a repeating string of characters.
Run-length encoding 9 Example: After RLE After RLE AAAABBBC 4A3B1C ENSAS 1E1N1S1A1S Gain = 25% Loss = 50% Rules for using RLE: Rule 1 : The character must be repeated at least three times. Rule 2 : If the sequence is not encoded, we above 00 followed by the number of characters . Rule 3 : If the sequence is odd, we copy 00 at the end of the sequences .
Run-length encoding 10 Different methods to encode images: There are a number of variants of run-length encoding. Image data is normally run-length encoded by uniform paving points, along lines, or even zigzag.
LZW (LEMPEL-ZIV-WEICH) 11 Definition : It is a method of compression dictionary based on reasons that are more often than others. Principle: Repeated sequences are stored in a dictionary and replaced by their address in the dictionary. The index is replaced by the sequence which is stored on a bit number smaller than the sequence.
LZW (LEMPEL-ZIV-WEICH) 12 Example : Size of image : 256*153*24 bits = 114 ko Compression LZW The size of the image : 51,9 ko Le taux de compression est de 2,21