jrn 440 adv. online journalism resizing and resampling monday, 2/6/12
TRANSCRIPT
About raster images
Raster images are output device dependent; you should know your target output device before you start your project
Another good thing to know is the dimensions of your file For print, how big will your raster image be on
the page 5” x 7”, 8 x 10” …
For web, how big will it be on a website 640 x 480 pixels
About raster images
You also want to set your resolution to be compatible to your output device
Resolution= Number of pixels in each unit of measurement Check your home printer, ask your commercial printer
(150 or 300 dpi for color and grayscale images, 600 dpi for black and white images)
Web or digital presentation (72, 96 ppi) Remember that if you are ultimately going to print
something, do not trust what you see on your monitor for resolution If you do or do not change the resolution, the image
will still appear sharp on the monitor
Attributes of Raster Images
Resolution Number of pixels in each unit of measurement (dpi,
ppi) Dimensions
Refer to number of pixels wide and tall (we say width and height)
640 x 480 is normally for a standard computer screen Can be any size you specify
Problem is that dimensions of screens are not standardized in new products yet
Think about screen sizes of various cell phones
What is dimension size limited to? Your patience, amount of storage space you have,
limitations /capabilities of capturing and output devices
Image Size vs. Canvas Size
Image Size Process of resampling (adding or deleting
pixels) Canvas Size
Changes the “paper size” around the image Also adds to overall size (memory) of file
Features in Image Size Dialog Box (<Image <Image Size) Pixel Dimensions The Image
Dimensions (width and height)
Under Document Size
Image Resolution Number of pixels
in each unit of measurement
Under Document Size
Features in Image Size Dialog Box (<Image <Image Size)
Image Size Mb, kb Why do you care about
this? Is your image going
to a website (may be downloaded)
Do you carry multiple images around on a jump drive?
Does your client want you to e-mail them an image?
Features in Image Size Dialog Box (<Image <Image Size)
Resizing and Resampling checkbox
Checked = resampling Unchecked = resizing
This little box is VERY powerful!
Resizing
Image Resizing: NOT changing the number of pixels Changing the size the raster image will print without changing the number of pixels in the image.
Changing the resolution, not the number of pixels
Overall file size (mb) does not change Think of it as a “tradeoff” between resolution
and dimensions
Resizing: How to do In Image Size Dialog
Box Uncheck the
Resample box Notice what gets
grayed or blocked out
If you have a large lo-res file, increasing your resolution will decrease your width/height But overall file size (mb) will remain the
same
Resizing your Image
UNCHECK the “Resample Image” box
Resizing let’s you make a trade off between resolution and dimensions
So if you have a large and lo-res image, you can also have a small and high-res image
Resampling
Image Resampling: Changing the number of pixels in the raster image. Can change it all, resolution, document width
and height/ pixel dimensions Overall file size (mb) does change Think of it as “silly putty”
Resampling: How to do In Image Size Dialog
Box Check the Resample
box Everything can now
be changed.
But, again, be careful! Your monitor won’t show you what’s happening to your file!
Ways to Resample your Image
Resampling: changing the # of pixels in an image <Image < Image Size What do you want to change?
Keep Resample Image Box checked Resolution- increase or decrease Document Size (think of it as print size) –
increase or decrease Changing either one of these also changes
overall file size (kb, mb) of image Which is confusingly placed on top of Image Size box next
to Pixel Dimensions
Ways to Resample your Image
Resampling: changing the # of pixels in an image <Image < Image Size What else do you want to change?
Keep Resample Image Box checked Pixel Dimensions
Are used in building images for web pages The display size of an image on-screen is determined by
the pixel dimensions of the image plus the size AND the setting of a monitor
15 inch monitor displays 800 pixels horizontally by 600 vertically
Changing this will also change the overall file size (kb, mb) of image
Calculations and Resampling
With Resample box checked If you change the resolution, you change the overall
file size. BUT, you keep the dimensions the same If you increase resolution, your file size will increase
by the square of the value Resolution (x) =File size (x2)
If I double my 72 ppi resolution to 144 ppi, the file size will quadruple (2 squared or 2 x 2)
If I triple my 72 ppi resolution to 216, the file size will be 9 times as large (3 squared or 3 x 3)
Details when Resampling your Image <Image < Image Size What do you want to change?
Keep Resample Image box checked Constrain Proportions box keeps the image
from being distorted Scale Styles box determines whether or not
Layer Styles are scaled along with the layers they modify.
Has no meaning if an image that does not use Layer Styles.
Up or Down Sampling
General rule- don’t upsample Why? Can’t add details that weren’t there in the first
place Silly putty comparison.
Downsampling is better since it throws away data Three methods (under ‘Resample Image’ box)
Nearest neighbor- fastest but least accurate Bilinear Bicubic- Creates best effects but takes the longest
3 Methods of Interpolation
When you resample, Photoshop fills in missing pixel data or replaces groups of pixels using a Mathematical approximation process called Interpolation
Nearest Neighbor Fastest and least accurate Copies the value of one pixel next to existing
one Then uses that value to create a new pixel
3 Methods of Interpolation
Bilinear Medium method in accuracy and speed Averages 4 neighboring pixels and then adds that
average value to the image. Then uses a less sophisticated algorithm to smooth the
transition between the added pixels and existing pixels Bicubic (3 versions of)
Creates best effects but takes the longest Averages the color or gray values of the 16 surrounding
pixels and adds that average value to the image. Then uses a calculation intensive algorithm to produce
the smoothest tonal gradation around the image Sharper for downsampling; smoother for upsampling
Image Size Dialog Box Generally Speaking Resampling images by changing the width
and height values in the Pixel Dimensions section of the Image Size dialog box is primarily used when optimizing images for the web.
Resampling or resizing images by changing the width, height and/or resolution values in the Document Size section of the Image Size dialog box is used for print.
Summary
Image resizing keeps the number of pixels in your image the same and affects only how large your image will print (the Document Size).
Image resampling physically changes the number of pixels in your image (the Pixel Dimensions).
The Resample Image option at the bottom of the Image Size dialog box controls whether you're resizing or resampling an image.
With Resample Image checked, you're resampling the image. With it unchecked, you're simply resizing the image.
It is always best to resample down.