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Sunday, May 31, 2009

Photography tips - How to compress large file format ?

When you capture image using digital cameras, the image file size is usually very large compared to many other image format stored in the computer. A low resolution 640 x 480 image has 307,200 pixels. If each pixel uses 3 bytes for true color, a single image will eat up a megabyte of your storage space. So as you increase the resolution size, the file size considerably increases. A 1024 x 768 pixel resolution at 24-bit true color (3 bytes per pixel), the image takes up 2.5 megabytes of your storage space. So to make the file size smaller and manageable, and for faster downloading and uploading process, image compression is applied using different compression tools. Digital cameras use its own form of compression.


How does compression work?

Digital photography uses two methods of compression, lossless and lossy. The former which is also called reversible compression uncompresses an image to match the original image source. During the process, data that are duplicated which has no value are elimanated producing a smaller file size usually a third from the original size. The latter on the otherhand does not uncompress images to the same quality as the original source but the images appears normal and the same. During the process, data that is not obvious to the viewer is removed. For example, if large areas of the sky are the same shade of blue, only the value for one pixel needs to be saved along with the locations where the other identical pixels appear in the image. Compression ratio ranges from 10:1 to 40:1. Lossy compression is commonly done when images are posted on the internet.

How to compress images?
Most digital cameras have built-in compression tools. You simply select the appropriate settings of your camera how the image is to be stored, either in high resolution or low resolution. In my case, since my storage space is large enough, I set my camera to the highest resolution it is capable of. I only compress the image file when I share it to my friends and upload to the internet. There are several comperssion tools available like adobe photoshop, microsoft picture manager. To compress the file without degrading much of its quality, you set the DPI to 72 and image size to at least 480 x 320. In such method a 2.1 megabyte image is reduced to 53 kilobyte size. You may also use the imager resizer for batch compression of images.

Do you see any difference of image quality between the two pictures?
The first image is the uncompressed 1024 x 523, 455kb file size while the second image is the compressed version 480 x 246, 65kb file size.

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