This document discusses raster images and resizing. There are two ways to resize a raster image: change the pixel size or change the number of pixels through resampling. Resampling creates new pixels by interpolating data from original pixels, so the resized image is not identical to the original. Resizing can introduce unwanted colors or effects. Bit depth determines the number of colors or shades of gray an image can display, with more bits allowing more colors or levels of gray. File compression is used to reduce large raster file sizes, and can be either lossless or lossy.

1. Raster Images & Resizing
 There are only two ways to resize a raster image.
 Change the pixel size (effective resolution)
 Change the number of pixels (resample)

2. Raster Image
 Photoshop™ can resample images. (change amount
of pixels)
 Resampling renders new pixels from old, by
interpolating original pixel data.

3. Raster Image
 Beware! All of the pixels are NEW.
 They are not the same as the original capture.
 “Artifacts” can be created by the
resampling/rendering process.

4. Raster Image Resampling
 Unwanted colors or effects may result from resampling

5. Raster Image
(change the number of pixels)
 Resampling
 Upsampling
 Add pixels to increase resolution
 Downsampling
 Delete pixels to decrease resolution
 Change size of an image and maintain the original
resolution

6. Raster Image Resampling
 Downsampling
 Good for reducing the file size of a large image
 Upsampling can increase the resolution of an
image with resampling.
 Can reduce jagginess in CT images
 Trade-off is image sharpness
 It will not help BIG enlargements.

7. Raster Image
Upsampling
No Upsampling Upsampled to 72 ppi
72 ppi 72 ppi

8. - Line Art
 It cannot make a jaggie bitmap image smooth!
 Capture at a high resolution initially
Line Art
100 PPI 200 PPI

9. Rasters & Rotation
 Rotating a raster image results in a different pixel
alignment caused by how the rotated image realigns to
the image grid (raster).

10. Rasters & Rotation
 Rotation triggers resampling.
 Straight edges can become jagged.
 Interpolation calculates the newly created pixels.
Original Image Location Rotated Image New Pixel Location

11. 
Bit Depth
Raster Image Characteristic

12. Raster Image Bit Depth
 The pixels will either represent black & white, shades of
gray, or colors.
 The data collected for each pixel is stored as bits of
computer data.
 A bit is the basic component of all computer data.

13. B&W - Grays
 Black & white pixels are stored with one bit of
data per pixel.
 The bit is either a 0 OR a 1.
0
Black
White

14. B&W - Grays
 More than one bit is needed for storing shades of
gray.
 Two bits of data per pixel stores four shades of gray
(black, dark gray, light gray, white).

15. B&W - Grays
Original Photograph
Grayscale
Four Levels of Gray =
2 Bits of data per pixel

16. B&W - Grays
 The number of bits per pixel as an exponent of 2 equals
gray levels.
Normal Grayscale
Is 8 Bits/Pixel
2
Number of bits
21 = 2 levels
22 = 4 levels
23 = 8 levels
24 = 16 level
28 = 256 levels

17. Digital Values
 With 8 bits, 256 different values (grays or colors)
are possible.
 We can count from 0 to 255.
0 = Black or no color
or maximum color

18. Digital Color
 Scanning red (R), green (G), and blue (B), a value is
captured for each pixel.
 Each R, G, and B value is captured and stored in 8 bits.
 24 Bit = 16.7 million colors

19. Digital Color
 Four Color Process is 32 bit
Does NOT reproduce as 400 million
colors

20. Digital Color
 16 Bit Images - High-Bit
 48 Bit Color (16 bits per color channel RGB)
 Camera Raw .CRW
 For editing only - Export at 24 Bit

21. Digital Color
 Indexed Color
 8 bit - 256 Colors (out of 16.7million)
 Applied to GIF format
 PNG-8
 Supports indexed transparency
16 x 16 = 256 Color Index

22. File Compression
 Large raster files often require compression
 Two basic types of compression
 Lossless - no data is lost in compression and
extraction
 LZW (TIFF, GIF)
 ZIP
 RLE (for line art)
 "Lossy" - data is lost to achieve greater
compression
 JPEG – compress once