Compression Tactics Used

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Compression Tactics used in PowerPressed

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PowerPressed has a range of user definable settings and options.

Presentation Compression

Given that PowerPoint® supports many image formats, it should be expected that if a presentation contains images that are already in a compressed format (i.e. JPEG, GIF, etc) less dramatic savings in file size may result. However, the abundance of options found within PowerPressed should allow a user to find a setting that balances quality and still significantly lowers the size of their presentations.

Compression Levels

There are three default compression levels (Low, Medium and High) and also a Custom level that allows a user to adjust all the compression options for their specific needs.

Tip.

To achieve the highest quality optimized presentation, we recommended that you use original uncompressed images when you construct your original presentation and then choose Compression Level ‘Low or Medium’. This generally produces a file size difference of approximately 80 to 90 percent without significant visual degradation.

Reduction in File Size (recommended)

This compression is best suited to presentations that contain:

•	high quality photos

•	images containing text

•	scanned text

This setting will produce the smallest file size but can introduce visible artefacts into the images when using the ‘High’ compression setting.

Image Smooth

This compression is best suited for presentations that contain:

•	images that have already been compressed (jpegs, etc.).

General Notes on Image Compression

Different types of compression have been developed to exploit the unique properties of image data. Two primary methods of compression for image data are DCT (Discrete Cosine Transform) and wavelet which can achieve more effective compression than is possible through the use of binary compression. Essentially, image compression takes advantage of the fact that an image can be altered to a certain extent and still be recognizable as the same image to the human eye. The altered/compressed image therefore conveys the same information or meaning as the original image.

PowerPressed uses a DCT-based image compression which operates by reducing a closely grouped set of pixels of a similar color to a set of pixels of the same exact color. By repeating this throughout an image, long patterns are formed by new blocks of same colored pixels, thereby making the image more suitable for compression. Image compression mechanisms are often configurable to control the point at which the algorithm determines if a group of pixels is similar enough to try to lump together into a single block. A very relaxed setting allows the compression to larger, more compressible blocks while stricter controls will tend to make smaller blocks. One can actually see this effect in certain images which are highly compressed. The compressed image will display “artefacts” appearing as blocky edges and seemingly random splotches of color.

This compression technique requires that a certain amount of information needs to be discarded from the image. This is called lossy compression because information is lost from the image during the compression process. process.