By Pegasus Imaging Corporation

Optimizing JPEG Images for the Web

JPEG is certainly the best compression type for presenting scanned and captured images on the Web. However, even experts find that with a little help they can make their Web pictures even smaller without sacrificing quality. JPEG is a lossy compression format and the key to getting the minimal file size is choosing the maximum amount of compression without affecting the picture quality.

The JPEG Wizard uses advanced new technology from Pegasus Imaging to minimize or completely avoid generational loss and the problem of under-compressing or over-compressing your images. For more information on Pegasus Imaging, please visit www.pegasusimaging.com.

Each time you recompress a JPEG image you add a little “generational loss". This might happen if you add text, if you rotate the image, if you edit it, or if you just want higher compression. After too many generations of recompression, your images may lose their fine detail.

What does The JPEG Wizard do?

The JPEG Wizard Online Optimizer uses JPEG compressed images as its source and can perform multiple operations:

a) JPEG Optimization to create more highly compressed JPEG images, based on the compression settings supplied.

AND /OR

b)Lossless Rotation operations on the image.

What is JPEG Optimization?

Until now, when an image had been compressed with JPEG compression, the only way to decrease its file size was to convert the JPEG image to uncompressed data and then perform a higher level of JPEG compression on the uncompressed data. This resulted in a smaller JPEG file than the original JPEG file, but the smaller file contained more pronounced JPEG artifacts than would have existed if the original image had been compressed with the higher level of JPEG compression.

Thus "generational loss" due to treating JPEG compressed data as if it were uncompressed data was introduced into the image.

The Pegasus JPEG Wizard avoids "generational loss" by treating the source image data as altered by JPEG compression. The JPEG compression of the original image is defined in the source JPEG image. The JPEG Wizard is supplied the information defining the desired JPEG compression in the compression parameters. The JPEG Wizard then operates directly on the compressed JPEG data to perform the conversion between the source JPEG data and the requested JPEG settings.

This avoids the "generational loss" problem and creates a JPEG image with compression ratios and image quality approximating the JPEG file obtained when uncompressed image data is compressed with the supplied compression parameters.

What is Lossless Rotation ?

Until now, JPEG images that were compressed with the wrong rotation would have to go through the "generational loss" problem described above in order to view them at the proper orientation. Even without creating smaller JPEG file sizes, the "generational loss" would occur.

Use the JPEG Wizard™ to reorient the image without "generational loss". The JPEG Wizard™ can work with the JPEG data directly and simply move the JPEG data within the JPEG file without performing the loss inducing step of decompressing the JPEG data.

By specifying a new orientation and setting each of the compression parameters to 0, the JPEG Wizard™ will output a JPEG image that is losslessly rotated.

What are the "Compression Parameters"?

The compression parameters control the amount and type of loss allowed in the JPEG compression of image data.

Luminance is the compression parameter that determines the amount of loss allowed to the grayscale component of each pixel in the image.

Chrominance is the compression parameter that determines the amount of loss allowed to the color component of each pixel in the image.

Both Luminance and Chrominance accept values between 0 and 255. Increasing values increase the amount of loss allowed in each component of the image and the compression ratio obtained.

Setting Luminance and Chrominance to 0 informs the JPEG Wizard to perform the minimum optimization of the image. This is useful if the desired outcome is a losslessly rotated image. In this case, set a new orientation value and Submit the image to view the losslessly rotated image.

What are Baseline and Progressive JPEG types?

In both baseline and progressive JPEG compression, the image is processed in 8x8 blocks, block by block from left to right, and top to bottom. In baseline encoding, all of the data for each of the pixels that describe the image is immediately encoded and stored in the compressed image file. In contrast, in progressive encoding, only a portion of the data for each of the pixels is encoded and stored in one of the "scans".

When decompressing a baseline JPEG image, the image data is decoded at final resolution from top-left to bottom-right, a strip at a time. When decompressing a progressive JPEG image, the image is decompressed a "scan" at a time from top-left to bottom-right a strip at a time. Each "scan" of the image contains more information about each pixel of the image. Thus, the image goes from fuzzy to clear as each "scan" is decompressed and displayed.

Why is JavaScript required?

The JPEG Wizard uses JavaScript to set the compression parameters and determine browser types and perform other operations. You can turn this on and off at will in your browser.

How can I do this on my own machine?

The JPEG Wizard exists as a standalone windows application. It does much more than the online JPEG Wizard, including regional compression, watermarking, hue/tint/brightness/contrast and thumbnails; all available as a batch operation. Try it now for free at www.pegasusimaging.com/jpegwizard.htm

I have a digital camera and have some portrait orientation images (from rotating the camera!), how do I rotate the JPEG image WITHOUT increasing the compression?

Set Luminance and Chrominance to 0 and Orientation to Rotate 90 (or 270).

Why is the output image sometimes of different pixel dimensions than the original?

JPEG Optimization (and all JPEG compression) is performed on blocks of 8x8 or 16x16 pixels. If an image is re-oriented it may have some pixels padded to a dimension to achieve this reorientation.

What are JPEG artifacts and why are they more pronounced without JPEG Optimization?

JPEG artifacts describes the visual effects of over-compressing an image with JPEG compression. JPEG compression is essentially performed by separating an image into small rectangles and loosely compressing each rectangle independently of the others. When these compressed rectangles are placed next to each other in the full image, the transition from one rectangle to the next can creates the appearance of a pattern of blocks. The appearance of these undesirable artifacts signal that the JPEG compression parameters are set too high to achieve the desired image quality.