Some of All You Ever Wanted to Know About Digital Images And Were Probably Afraid to Ask Suzanne Paq

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Some of All You Ever Wanted to Know About
Digital Images And Were Probably Afraid to
AskSuzanne PaquetteJournal of
Histochemistry Cytochemistry
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Overview

• Types of Images and Image Files
• Halftones
• Vector Graphics and Line-art
• Rasterization
• Combination Images
• Images in Print
• Images and Colorspaces
• What should you send to your publisher?

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Types of Images and Image Files

• Halftones
• Digital image definition Halftone images are
  stored as a series of pixels and color
  information.
• Printers definition An image where no text or
  line drawings are set against a non-colored
  background.
• Halftone image files have a fixed number of
  pixels when they are saved.
• If you raise the size of a halftone but dont
  lower the resolution, or raise the resolution but
  dont lower the size, you will have to add to the
  total number of pixels.
• This will dilute the image quality.

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Halftone Image Examples
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Types of Images and Image Files

• Common Halftone file types
• JPEG (JPG) Low file quality, very small and
  good for the World Wide Web and email attachments
• GIF - Better than JPG for line drawings and text
  but poor with color management
• Bitmap (BMP) Higher quality, can be compressed
  using Stuff-It and WinZip
• TIFF (TIF) Very high quality, often very large
  files, can be compressed when saved using LZW
  compression.

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Types of Images and Image Files

• Vector Graphics (Line-art)
• Digital image definition True Vector Graphics
  and Line-art files are stored as a series of
  vectors and information for producing the image
  when it is opened for display (not as pixels!).
• Printers definition An image where text and
  line drawings appear against a non-colored
  background.
• Graphs, histograms, and diagrams are all
  considered line-art.
• Examples

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Line-Art / Vector Graphic Image Examples
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Line-Art / Vector Graphic Image Examples
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Types of Images and Image Files

• True Line-art files are scaleable
• This means they can be made larger without loss
  of image quality (no dilution).
• Why? They don't have a fixed number of pixels in
  their primary file format, just vectors.
• If you want the resolution or image size to be
  larger, new pixels can be made without diluting
  the image quality.
• If a resolution, size, and/or number of pixels
  has been applied to a vector graphic, it is said
  to be rasterized.

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Types of Images and Image Files

• Vector Graphics / Line-art File Types
• Encapsulated Postscript (EPS) and Postscript (PS)
• Portable Document Format (PDF) - Proprietary
  Adobe format
• Enhanced Metafile (EMF) and Windows Metafile
  (WMF) - Microsoft-specific clipart type file that
  can be used to store vector graphics.
• When a Vector Graphic / Line-art image is saved
  in a Vector Graphic File type, it is scaleable,
  and can be rasterized.

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Types of Images and Image Files

• Rasterization
• The process of applying a size and resolution
  (number of pixels) to a scaleable image (eg. A
  vector graphic).
• Rasterizing a vector graphic produces a halftone
  image.
• TIFF, Bitmap, JPEG, and GIF are all rasterized
  file formats, and cannot be used for storing
  scaleable Vector Graphics / Line-art.
• If a Vector Graphic is saved in a rasterized
  format, it is now a halftone image and is no
  longer scaleable.

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Types of Images and Image Files

• Combination Images (Combos)
• Images containing both halftone images and vector
  graphics / line art are called combination
  images or combos.
• This includes images where labeling has been
  placed on a white background outside a halftone
  image.
• Halftone images embedded in a vector graphic
  still have a resolution and a size they are not
  scaleable!
• The vector graphics / line art portions are
  scaleable as long as the combo has not been
  rasterized.

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Combination Image Examples
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Types of Images and Image Files

• Four Rules for Combo Images and Files
• When a combo is saved in a Vector Graphic file
  format, the line art portions of the file are
  still scaleable, and the halftone portions are
  not.
• If a combo is saved in a halftone file format, no
  part of it is scaleable.
• When you rasterize a scaleable combo file, the
  resolution and size of the embedded halftone
  images will be changed to conform with your
  settings.
• If you try to rasterize at a size that would
  require the halftone to be larger than the number
  of pixels available, your halftone may look fuzzy
  or indistinct.

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Combo Image Raster Examples
Fuzzy/indistinct reproduction of the halftone
from dilution
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Images in Print

• Print
• Publishers often use printing presses that will
  print at very high resolutions. Your files need
  to have high resolution to look good when
  printed.
• For halftone images, 200 DPI at the size intended
  for publication is the absolute minimum, and 300
  DPI gives the best results.
• For rasterized line-art and combos, 600 DPI at
  the size intended for publication is the absolute
  minimum, and 1000 DPI gives the best results.
• If you do not rasterize your vector graphics or
  combos, the only resolution you need to keep in
  mind is the halftone in the combo.

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Images and Colorspaces

• A colorspace is a set of colors.
• Gamut is the range of a colorspace.
• RGB
• Reasonably large gamut.
• Monitors and TVs use RGB
• CMYK
• Significantly smaller gamut than RGB.
• Many printers, especially printing presses, print
  in CMYK.

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Gamuts of RGB and CMYK
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Images and Colorspaces

• RGB to CMYK conversion
• Because RGB's gamut has many colors that CMYK's
  does not, converting from RGB?CMYK results in
  loss of color.
• This can 'flatten' the look of the image, or make
  it seem dull.
• If you convert from RGB?CMYK and save the
  changes, you have permanently lost any colors
  outside the CMYK gamut.

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RGC?CMYK Conversion Example
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What should send to your publisher?

• Avoid sending program-specific,
  platform-specific, or proprietary file formats
• The publisher may not have
• The program
• The appropriate version of the program
• The appropriate platform (operating system)
• Anyone who knows how to work with the file type.
• This includes but is not limited to
• Jasc PSP (Paint Shop Pro) files
• Adobe PSD (Photoshop), INDD (InDesign), AI
  (Illustrator), and P65 (PageMaker 6.5) files
• CNV (Canvas) files
• Corel CDR (Corel Draw) files

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What should send to your publisher?

• Try to send generic file formats
• Most programs can support them by default.
• The version of the program that made/edited the
  file will not usually determine if another
  version can edit it.
• Often recognized by non-image-editing software.
• Generic image file types include TIF, Bitmap
  (BMP), Encapsulated Postscript (EPS), Postscript
  (PS), JPG, and GIF
• One exception Adobe Acrobat PDFs are usually
  recognized by non-Adobe software.

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What should send to your publisher?

• Publishers often list file types they support
• Look over this list before sending a proprietary
  file format.
• Take note of any versions or special notes.
• Send multiple file types if you want to be sure.
• Send high resolution files
• There is no such thing as too much resolution.
• There is such a thing as not enough resolution.
• Only two solutions for low resolution images
  Dont publish them, or let them look bad when
  published.

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What should send to your publisher?

• Consider sending color images in CMYK
• You may have no control over what changes are
  made in the conversion if you send RGB.
• The publisher may have to come back to you and
  confirm you are alright with the color loss in
  conversion.
• Look at an RGB?CMYK conversion of your color
  figures
• You will have a better idea of what they will
  look like in print.
• No confirmation needed from the publisher you
  know what to expect.