In physics, compression means that inward forces are evenly applied to an object from different directions. During this process, the atoms in the object change their position. After the forces are removed, the object may be restored depending on the type of materials it is made of. For example, when compression is applied to an elastic material such as a rubber ball, the air molecules inside the ball are compressed with decreased volume. After the compression force is removed, it quickly restores to its original sphere shape. On the other hand, when a compression force is applied to the brick, the solid clay cannot be compressed. Therefore, the compressive forces concentrate on the weakest point, causing the block to break at the middle point.
For images, compression is the process of encoding digital image information by using a specific encoding scheme. After compression, the image will have a smaller size. An image can be compressed because of the degree of redundancy. Since the neighboring pixels of an image are correlated, the information may be considered redundant among the neighboring pixels. During compression, these redundant pixel values (i.e., values close to zero when encoding digital image information) are removed by comparing with the neighboring pixels. The higher the ratio is, the more the small values are removed. The image after compression uses fewer bits than the original unencoded representation, therefore, achieving size reduction purposes.
In the area of medical imaging, there are two different methods of compression that are commonly used, JPEG and JPEG2000. JPEG2000 is a new image coding system that compresses images based on the two-dimensional discrete wavelet transform (WDT). Unlike JPEG compression which decomposes the image based on the frequency content, JPEG decomposes image signals based on scale or resolution. It performs better than JPEG with much better image quality at moderate compression ratios. However, JPEG2000 does not perform well at a high compression ratio with the image appearing blurred.
Now onto the fun part, using compression in a sentence by the end of the day!
Serious: This high compression ratio has caused so much information loss that I cannot even recognize that it is a dog.
Tong: I could do compression with my eyes.
Grace: Really? How?
Tong: It is simple. Just remove the eyeglasses. Everything becomes compressed.
See you in the blogosphere!