1. NIMAS Images and File Size Julia Myers Nicole Gaines January 29, 2008

2. Image File Size and Image Quality Image file sizes, expressed in bytes, increase with the number of pixels in the image, and the color depth of the pixels. Although from a pragmatic perspective the accumulated storage represented by the images in NIMAC depository items presents challenges in how the materials are transmitted, preserving the appropriate size and resolution of images continues to be important. The size of any individual image in itself does not pose as much of a problem as the difficulty of transmitting packages that may include hundreds or even thousands of images.

3. Resolution, Resolution, Resolution! Images should be a minimum of 300 DPI for print and 400 DPI if an image includes text. Photographic printing typically occurs at 1200 DPI. Web images should be 72-120 DPI, which is far below standards for print media. 72 DPI corresponds to the resolution of most PC monitors. DPI=Dots per inch

4. More about resolution … How an image was ORIGINALLY acquired determines its resolution. Resolution and size are inversely proportional. Enlarging an image lowers resolution.

5. Image File Size and Image Quality Lossless compression Lossless compression algorithms reduce file size with no loss in image quality, though they usually do not compress to as small a file as a lossy method does. Lossless compression can rarely do much better than 2:1 compression.

6. Image File Size and Image Quality Lossy compression Lossy compression algorithms allow for variable levels of quality (compression) and as these levels are increased, file size is reduced but quality deteriorates. This deterioration is known as compression artifacting.

7. NIMAS Image Formats: JPEG JPEG is a lossy format. JPEG stands as the most widely used image format, especially for images used on the Web. JPEG uses an algorithm that results in some irretrievable loss of data. JPEG can be applied at varying levels. More aggressive application of JPEG compression results in smaller file sizes, but more loss of information. In most cases the loss is technical. Even with data loss, degradation of the visual perception of the image can be very minimal.

8. More about JPEG … JPEG can produce a smaller file than PNG for photographic (and photo-like) images since it uses a lossy method specifically designed for photographic image data.

9. Improved JPEG Formats There are 3 lossless or near lossless JPEG formats: Lossless JPEG, JPEG LS, JPEG 2000. Of the 3, JPEG 2000 is more widely supported but offers worse compression ratios.

10. JPEG 2000 JPEG 2000 uses a different compression, based on wavelets, that has many benefits over the original JPEG specification. JPEG2000 uses a lossless method, which limits the file size reduction options. JPEG 2000 is gaining a wider level of adoption. Motion JPEG 2000 is increasingly used for video rather than the traditional MPEG-2 approach.

11. NIMAS Image Formats: PNG PNG is a lossless format. PNG (Portable Network Graphics) has come into wider use in the last two years. PNG applies a less aggressive compression and does not result in the technical loss of data. Given that the algorithm does not allow data loss, the maximum level of compression results in larger file sizes than is possible with JPEG images, which can be compressed to extremely small file sizes, but with ever increasing loss of image data and visual quality.

12. PNG vs. JPEG PNG files seem large when compared to JPEG files that have been aggressively compressed at levels that lose data to the point where they are visually apparent. However, using PNG instead of a high-quality JPEG for some images can result in a large increase in file size (often 5–10 times) with negligible gain in quality.

13. More about PNG PNG is a better choice than JPEG for storing images that contain text, line art, or other images with sharp transitions. PNG useful for saving temporary photographs that require successive editing. After editing, the image can then be saved as a JPEG, and limiting image quality loss to just one generation.

14. NIMAS Image Formats: SVG SVG is not a raster graphic. Scalable vector graphics are XML based and compress well. NIMAC has not received any of these image files to date, but would anticipate smaller file sizes with SVG. SVG scales well without image quality loss, but work continues to have full SVG native support in all major browsers.

15. Creating Image Files Many factors come into play when converting graphics files into compressed images. The original images may differ according to the native resolution and the level of color data. JPEG images tend to be smaller because the tools used for creating the images allow the user to choose among levels of compression.

16. Image File Size Implications: Size Matters Images, their number and size, drive NIMAS File size. NIMAS File Sets are commonly 2-3 GB in size and may reach 12 GB in size--or more. NIMAS Textbook Files contain thousands of images. File size can mean slow downloading, even for those with fast connections. Format matters: PNGs are typically 5-10 times larger than JPEG. NIMAS File size in context: typically a 90-minute movie is less than 1 GB. (This is due to compression method used for MPEG.)

17. Impact on NIMAC Users File sizes are currently a considerable obstacle for users attempting to download files. PNG images can be up to 10 times larger than JPEG files, yielding file sets that are several GB in size. PNG does not appear to provide significant advantages over JPEG given current production methods for large print and tactile graphics.

18. 300 DPI and Large Print Production The only useful "300 DPI" image for print production is one that was created/saved originally at that resolution. Upsampling or saving lower DPI in higher resolution degrades image quality. Saving a publisher provided low resolution image that may be sufficient quality in regular print so it can be reproduced in large print does not produce a quality image, even if resaved at 300 DPI.

19. NIMAC Recommendations SVG will continue to be the preferred format; however, 300 DPI JPEG will be preferred over PNG for the time being. PNG will still be accepted by NIMAC. PNG images should not be converted to JPEG if PNG is the format saved from the native program.

20. How to Reach Us http://www.nimac.us 502-899-2230 1-877-526-4622 (1-877-52-NIMAC) NIMAC@aph.org