Working With Images April 21, 2005
Why this workshop? • To end the frustrating hours spent trying to print posters. • To maximize the quality of your posters, manuscripts, PowerPoint presentations and web images. • To inject some good information into the informal support network.
Agenda • Image basics • Resolution • Color management • File formats • Printing • Incorporating images into documents
Step 1: Forget everything you know about working with graphics! … unless I ask a question
Graphic Types • Two fundamental types of graphics • Vector graphics • Raster graphics (a.k.a. bitmaps)
Vector vs. Raster Vector: Draw a 1024 x 768 pixel rectangle and fill it with light yellow. Draw a 885 x 628 rectangle with no fill and a 6pt black stroke. This makes a 504K Illustrator file. Raster: Make a 1024 x 768 pixel bitmap with a bunch of light yellow pixels and a few black pixels. This makes a 2.25MB image.
Graphic Types • Two fundamental types of graphics • Vector graphics • Raster graphics (a.k.a. bitmaps) • Graphics software used to do one type or the other. That is no longer true. • You need to pay attention to which type of graphic is being used and the conversion from vector to raster.
Vector Graphics • Consist of directions for drawing objects • File size depends on number and complexity of objects. • Do not have a “resolution” so they can be scaled without affecting image quality • Traditionally produced by drawing applications like Illustrator or Canvas
Vector Graphics • Best used for images consisting of geometric shapes and lines (i.e. maps, charts, stratigraphic sections, etc.) • Text is generally a vector graphic. • It’s hard to get into too much trouble with vector graphics except when you convert from vector to raster. • Beware of overly complex digitized maps. • Beware of ArcGIS exports!
Raster Graphics • Image mapped out by grid (raster) of pixels.
Raster GraphicsBit Depth • Each pixel is represented by 1 to 64 bits depending on color mode. Bitmap Grayscale RGB CMYK 1 bit 21 = 2 values 0 = black 1 = white 8 bits 28 = 256 values 0 = Black 255= white This gray is 119 24 bits 224 = 16,777,216 8 bits for each channel (R,G & B) This is 255, 153, 0 32 bits 232 = 4,294,967,296 8 bits for each channel (C,M,Y & K) This is 0,44,95,0
Image (Color) Modes • PhotoShop (and other applications) can work in numerous image modes. • Grayscale, Indexed, RGB and CMYK are the most common. • Use grayscale for “black and white” images. • Use indexed mode when creating GIFs. • Use RGB for all other images.
Raster GraphicsImage size • Image size (bits) = pixel dimension X bit depth • Divide by 8192 to get Kilobytes • Increasing the number of pixels geometrically increases the image size. • 500 x 500 24 bit image ~ 732K • 1000 x 1000 24 bit image ~ 2.9MB • 2000 x 2000 24 bit image ~11.4MB
100 x 1008/9s of original pixels discarded 300 x 300Original Raster GraphicsImage scaling • Raster graphics are not scalable. Changing image size requires adding or removing pixels. 600 x 6003 new pixels created for each original pixel
Raster GraphicsFile size • File size does not usually equal image size. • Many file formats, such as JPEG and GIF, employ compression that shrinks the file size below the image size. • Example: a 1024 X 768 red box is 2.25MB, but saved as medium quality JPEG the file shrinks to 4.8KB; GIF is 1.5KB.
Critical take-home point #1 • No matter what size you shrink the file to, the image size stays the same! Critical take-home point #2 • It’s the image size that matters when printing or displaying images! Compressed files such as a JPEGs are uncompressed to the full image size before they are used.
What is resolution? • It depends on whether you are talking about an image’s specification, a printer’s print quality, a scanner’s hardware, a monitor’s pixel dimension or a digital camera’s CCD. • Resolution is one of the most misunderstood and misused terms in graphics.
Why is resolution important? • Properly specified image resolution is critical to print quality, printing speed and printer stability. • Improperly specified resolution can lead to all sorts of trouble (i.e. waiting 2 hours while the printer decides not to print your document).
Device Resolution • For printers it specifies the number of ink spots that can be printed per unit area • For scanners it specifies the physical spacing of optical sensors • For monitors it specifies the total number of displayed pixels • For digital cameras it specifies the total number of light-receptive cells in its sensor
Image Resolution • Image size has no physical reality; it just specifies the total number of pixels which have no inherent size. • When an image is displayed or printed it takes on a physical size. • Image resolution controls the conversion between the image size and the physical display size.
The Acronym Game • DPI: Dots Per Inch • PPI: Pixels Per Inch • SPI: Samples Per Inch • LPI: Lines Per Inch (a.k.a. screen frequency)
Display/Monitor Resolution • The dpi resolution specified in image files has no impact when they are displayed on computer monitors or data projectors. • Monitors can only display at their set resolution. • Each pixel in the image file is mapped to a pixel on the monitor.
Which image is 20dpi and which is 600dpi? It does not matter because they are both 500 x 350 pixels! 600dpi 20dpi
Proper Image Dimensions for Display • Figure out the size of your display device. 1024 x 768 is most common these days. • Size your images relative to display size • 1024 x 768 = full screen • 874 x 618 = image with ~1 inch border • 512 x 384 = 1/4 screen • Consistent image size adds a look of professionalism. Develop a set of preferred sizes and stick with them.
What about 72/96 dpi? • In the old days the common monitor at native resolution had an effective resolution of 72dpi. Today it varies. • Example: The display width of my 17-inch CRT monitor is 12.25”. In 1024 x 768 mode this gives an effective resolution of ~84dpi. • Forget about 72/96dpi—it’s meaningless when displaying images.
Scanner ResolutionSamples Per Inch (spi) • The native resolution of a scanner is determined by the spacing of sensors in its scanning bar. • UMAX 1100 in Image lab = 1200spi • Epson Perfection 2400 Photo in Mac lab = 2400spi • Minolta Dimage slide scanner = 2400spi • Scanning at the native resolution gathers the maximum amount of info from the original document.
Scanner ResolutionSamples Per Inch (spi) • Always scan at the native resolution and then let PhotoShop downsize the image if needed. • Exception: If you are in a hurry and you know your image is destined for low resolution use like the web or PowerPoint presentations, then scan at lower resolution for increased scanning speed. • Never scan at a setting higher than the native resolution. This causes the scanner software to make up pixel data. Let PhotoShop do the upsizing.
Printer Basics • Images are printed as a series of dots. Your brain interprets the dots as continuous tones. • The dots are solid colors (black, cyan, magenta or yellow). • The resulting color on the page is a function of dot concentration and the ratio between the different colored dots. • The conversion of your image into the series of dots is halftoning.
Example of Halftones Enlargement illustrates what halftone would look like if this image was printed on the color laser writer or plotter.
Printer ResolutionDots Per Inch (dpi) • Only applies when printing a monochrome (one color) image. • Text or 1-color line art (black, cyan, magenta or yellow). • Completely irrelevant when printing all other images. • I repeat, completely irrelevant when printing all other images!
Printer ResolutionLines Per Inch (lpi) • Also known as line screen or screen frequency • Specifies the number of halftone cells (spots) printed per inch • The LPI is much lower than the DPI and is generally hard to determine because printer manufacturers do not advertise it.
Printer ResolutionThe importance of LPI • Image quality peaks when the resolution is set to ~2 times the line screen. • Many images look just fine at 1.2 - 1.5 times the line screen. • Sending images at resolutions higher than 2.5 times the line screen degrades image quality and clogs the printer with needlessly large files.
Proper Image Resolution for Printing • Use 100- 140dpi for normal images lacking fine detail or where the fine details are not critical. • Bump up to 140-200dpi when the fine details count or you are printing small images. • Go up to 200-255dpi when printing small detailed images. • Never use a resolution >255dpi. • Experiment with resolutions <100dpi on low-detail images such as backgrounds.
Impact of Proper Image Resolution • Let’s take the example of a color image we want printed at 10 x 8 inches • At 140dpi the image is 4.5MB • At 200dpi the image is 9.4MB • At 600dpi the image is 84.4MB • The 600dpi file is 19x larger than 140 dpi and 9x larger than the 200dpi file.
Impact of Proper Image ResolutionThe Extreme Case • Let’s take the extreme example of a color image 12” tall spanning a 5’ poster • At 120dpi the image is 30.4MB • At 160dpi the image is 54MB • At 600dpi the image is 759.4MB • 120dpi is fine for almost any image you’ll print at this size. 160dpi is plenty for even finely detailed images. 600dpi makes the plotter go BOOM!
Critical take-home point #3 • Less is more when setting image resolution. Never use an image resolution higher than than you need!
Sizing Images in PhotoShop Use these boxes when sizing for display Use these boxes when sizing for printing Turn Resample Image off when you want to change resolution without changing image size Use Bicubic Smoother when upsampling and use Bicubic Sharper when downsampling
Color Management There be monsters here…
What color is this? Purple? Nope. It’s 100110000001001111111111
The Monsters of Color • Computers cannot see color. To them images are only a collection of 1s and 0s. • Monitors cannot see color. To them images are just electrical signals that tell them how to adjust electron flow. • Printers cannot see color. To them images are only a set of directions on how to control printheads.
Red, Green, Blue Used by monitors and data projectors Works by adding colors together (additive) Red+Green+Blue = White Cyan, Magenta, Yellow, Black Used by printers Works by absorbing color (subtractive) Cyan+Magenta+Yellow =Black Imperfections in ink absorption causes C+M+Y= muddy brown Black (K) is added to fix the problem The RGB vs. CMYK Complication
The Reality of Color • Image color is device-dependent. It varies depending on the physical characteristics and configuration settings of the computer, monitor and printer. • Example: In a store the TVs are often all tuned to the same program. They are all receiving the same input signal yet they all display slightly different colors.
Color Management • Color management is the science of displaying and printing images with consistent color across different computers and printers. • It’s as mysterious as theoretical physics, but fortunately we can get by without knowing too many of the details.
Color Spaces • Color spaces are 3D models used to represent a range of real-world colors using numbers. • Each color space contains a different sub-set of real-world colors (check out the Color Sync utility). • Color profiles define the color space and are the basis for all color management operations.
Working Space vs. Device Spaces • The Working Space Profile defines the color space within the application. • You must specify the working space. • In Illustrator, PhotoShop, and InDesign each image has its own working space profile. • Device Space Profiles define the color behavior of the device (monitor, printer, scanner, digital camera, etc.). • For most devices you just select the proper manufacturer’s profile. • However, you should create your own monitor profile.
Working Space • Keep images in RGB mode • Specify the sRGB color space (sRGB IEC 61966-2.1) • This color space closely matches the color range displayed by monitors. • The color laser printer and both HP plotters are configured to color match images created in the sRGB color space.