Image Compression

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Image Compression

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In the process of conversion, Modern PDF automatically compresses text, line art, and resamples monochrome, grayscale, color images. Depending on the settings you choose, compression and resampling can significantly reduce the size of a PDF file with little or no loss of detail and precision.


Available Options:


Compress graphics in PDF option is included in this PDF Page settings tab, and it allows you to compress images and graphics in PDF document and make it as smaller as possible.


Convert images to JPEG format allows you to convert all the images used in your document to JPEG format, and this would help reduce the output PDF file size.





Compression Algorithm:


ZIP is a compression method that works well on images with large areas of single colors or repeating patterns, such as screen shots and simple images. The ZIP method is lossless, which means it does not remove data to reduce le size and so does not affect an image's quality.


The JPEG (Joint Photographic Experts Group) compression method is suitable for grayscale or color images, such as continuous-tone photographs that contain more detail than can be reproduced on-screen or in print. JPEG is lossy, which means that it removes image data and may reduce image quality, but it attempts to reduce file size with the minimum loss of information. Because JPEG eliminates data, it can achieve much smaller file sizes than ZIP compression.


The JPEG2000 is a wavelet-based image compression standard. It was created by the Joint Photographic Experts Group committee in the year 2000 with the intention of superseding their original discrete cosine transform-based JPEG standard (created about 1991). JPEG 2000 requires far greater decompression time than JPEG and allows more sophisticated progressive downloads, yet averages similar compression rates. JPEG 2000 becomes increasingly blurred with higher compression ratios rather than generating JPEG's "blocking and ringing" artifacts, complicating direct comparison of their respective compression rates. Images machine-judged to be of equivalent quality for both compression schemes often look better to humans in JPEG 2000 at low bitrates.


Modern PDF automatically detects the image type, quality, format and decide the best compression algorithms.


Image Quality:


Modern PDF provides five JPEG options, ranging from Maximum quality (the least compression and the smallest loss of data) to Minimum quality (the most compression and the greatest loss of data). The loss of detail that results from the Maximum and High quality settings are so slight that most people cannot tell an image has been compressed; at Minimum and Low, however, the image may become blocky and acquire a mosaic look. The Medium quality setting usually strikes the best balance in creating a compact file while still maintaining enough information to produce high-quality images.


The CCITT (International Coordinating Committee for Telephony and Telegraphy) compression method is appropriate for black-and-white images made by paint programs and any images scanned with an image depth of 1 bit. CCITT is a lossless method.


Modern PDF provides the CCITT Group 3 and Group 4 compression options. CCITT Group 4 is a general-purpose method that produces good compression for most types of monochrome images. CCITT Group 3, used by most fax machines, compresses monochrome images one row at a time.


Run Length is a lossless compression option that produces the best results for images that contain large areas of solid white or black.


Resampling Method:


Resampling refers to changing the pixel dimensions (and therefore display size) of an image. When you downsample (or decrease the number of pixels), information is deleted from the image. When you resample up (or increase the number of pixels), new pixels are added based on color values of existing pixels. You specify an interpolation method - average downsampling, bicubic downsampling, or subsampling - to determine how pixels are added or deleted.


To resample an image, Modern PDF combines pixels in a sample area to make one larger pixel. You provide the resolution of your output device in dots per inch (dpi), and Modern PDF combines pixels as needed to reduce the image's resolution (ppi) to the specified dpi setting:


Average downsampling averages the pixels in a sample area and replaces the entire area with the average pixel color at the specified resolution.


Bicubic downsampling uses a weighted average to determine pixel color and usually yields better results than the simple averaging method of downsampling. Bicubic is the slowest but most precise method, resulting in the smoothest tonal gradations.


Subsampling chooses a pixel in the center of the sample area and replaces the entire area with that pixel at the specified resolution. Subsampling significantly reduces the conversion time compared with downsampling but results in images that are less smooth and continuous.


Modern PDF automatically detects the image type, quality, format and choose the most suitable resampling method.