Transcript Slide 1

NIMAS Images and File Size
Julia Myers
Nicole Gaines
January 29, 2008
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.
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
More about resolution …
• How an image was ORIGINALLY acquired
determines its resolution.
• Resolution and size are inversely proportional.
Enlarging an image lowers resolution.
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.
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.
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.
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.
“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.
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.
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.
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.
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.
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.
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.
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.)
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.
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.
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.
How to Reach Us
• http://www.nimac.us
• 502-899-2230
• 1-877-526-4622 (1-877-52-NIMAC)
• [email protected]