Various Compression Types
Adobe PDF compression can be utilized to reduce the size of PDF files, so that no errors occur while uploading or downloading them online. To convert the PDF files, Adobe PDF compression can be used to effectively compress and resample monochrome, grayscale and color images without any loss of details. The Adobe PDF online compress offers ZIP, JPEG and CCITT compressions to abridge line art, color images and monochrome images respectively. ZIP compression shows convincing results for images which have repeating patterns or large portion of single color like screen shots created using paint programs. This ZIP filter does not disturb the quality by removing data from the image for compression and is hence called lossless compression. For more detailed color images, JPEG compression is suitable. Although it is lossy, it can achieve smaller PDF files than any other compression. Adobe PDF compression offers maximum and minimum quality JPEG compression which will remove data accordingly. The lossless CCITT compression is desired for black and white graphics. Adobe PDF compression grants group 3 and 4 options which are utilized by fax machines and for sound compression of monochrome images.
Methods of Compression
The pixel dimensions are altered, resulting in subsequent decrease in size of the image and this process is referred to as resampling. To nub the size of the image, certain pixels can be removed, that is, certain information can be deleted from the image. This decreases the quality of that image and is called down sampling. Resample up or add appropriate pixels by considering the color values of the other pixels to compensate the loss due to down sampling. Adobe PDF compression provides outstanding features for this process to be performed. It cannot resample up the images but can combine smaller pixels to produce larger ones instead. The desired resolution of the output can be specified in terms of dpi for Adobe PDF compression to concise the image size according to the mentioned resolution.
To determine the addition and deletion of pixels, it is required to specify one of the following interpolation methods: average downsampling, bicubic downsampling and subsampling. At the mentioned resolution, an average pixel color is used to replace the sample area in the first method. Bicubic downsampling is the most circumscribed method producing smooth tonal gradation but is extremely slow. Although the images resulting from subsampling are not smooth and continuous, the conversion time is lessened significantly.