Digital Image

Bitmapped (raster) image

Collection of pixels (dots) laid out in a big grid
Pixels can be different colors
Scanners, video cameras, digital cameras, painting & image-editing programs (Adobe Photoshop)

Notes

Dimension

Dimension -- number of pixels in width and height.

600x400 pixels
More pixels, more information

Lets say you have a 600 pixel x 400 pixel image. What is the size of the image in inches?

Notes

Resolution

Depends on how the dots are spread out.

300 PPI
PPI: pixels per inch (resolution of the image).
Same dimension (4 x 4 pixel image) with different resolution (1 ppi or 4 ppi)

Notes

Dimention + Resolution

Dimensions -- amount of information/data you have (eg. 600 x 400 pixels)

Number of pixels == proportional to file size (KB, MB, GB, TB)
(more pixels you have, larger the file, more information you have).

Resolution -- how the pixels are spread out. (eg. 300 ppi)

It does not tell you how much information you have
It does not describe the quality of the image by itself.
Saying that "an image is 4000 ppi" is like saying "that there are 12 inches in a foot".
It doesn't tell you how long something is.

For the purpose of this class, we are going to use 300 ppi for editing and printing, as our printers expect images at 300 pixels per inch.

Most images on the Internet are not print quality!

600 x 400 pixel at 300 ppi ==> 2 x 1.33 inches

600 x 400 pixel at 1 ppi ==> 600 x 400 inches

have the same amount of information.

Notes

Input

Notes

Output (printing/display)

Notes

How much to scan?

Depends.......on your final output.

8 x 10 inch print on a 300 dpi printer

You need 2400 x 3000 pixels (approx. 18 MB)

Notes

File Formats

Lossless compression, large file size but no loss of quality

TIFF (.tif, .tiff)
Photoshop (.psd)
- When you are actively editing your file, save as a TIFF file or a PSD file.
- Understands Layers
PNG (portable network graphics)
- Mostly used for image on web. Simple with little options.
- Supports transparency
GIF
- Resurgence with animated GIF

Lossy compression, small file size with loss of quality

JPEG/JPG (.jpg, .jpeg)
Do not edit in JPEG. Every time you save in jpg, you lose data. The only time you want to save in JPEG is when you are distributing your files.

Lossless compression (168KB):

Lossy compression (84KB):

Notes

Resampling

What happens when the number of pixels you have isn't enough for your target print size?

Photoshop can make up/throw away information

Resampling = changing the number of pixels

Upsampling (enlargement): making up info. You get more pixels. But don't expect miracles.
For example, to upsample 600x400 pixels to 1200x800 pixels, Photoshop does a lot of math to fill in the missing pixels.
Downsampling: throws away info.

Upsampling 2 pixel image to a 3 pixel image


How about this?

Notes

Image Size in Photoshop: Resample ON

In Photoshop: Image -> Image size -> Resample image button

You are making up pixels (upsampling) or throwing away pixels (downsampling) when you have resample turned on

Upsampling (making up pixels): 600x400 pixels =========> 6000x4000 pixels
Downsampling (throwing away pixels): 600x400 pixesl =========> 60x40 pixels

Exercise:

1. Take a 600 x 400 pixel file and downsample it to 6 x 4 pixel file
2. Then take that 6 x 4 pixel file and upsample it to 600 x 400 pixel using Nearest Neighbor (from pulldown menu that says "Automatic")
3. Repeat (with the unmodified original 600 x 400 pixel file) steps 2 using different algorithms

What happens if you don't have the resampling button on?

Notes

Image Size in Photoshop: Resample OFF

600 px x 400 px, 703.1K

    2 x 1.333 inches @ 300 pixels/inch
    
    60 x 40 inches   @ 10 pixels/inch

    600 x 400 inches @ 1 pixels/inch

Still 600 px x 400 px, 703.1K....

Notes

Understanding the numbers used in digital imaging (not just photoshop)

Image -> Adjustment -> Levels
Generally, a good quality image has wide range of colors (from darks to lights)
Don't trust your eyeballs too much. It's only a reference.
- What we see is not necessarily what we have (notice the difference in color of all the TV monitors when you go to an electronic store).
- Output devices (eg. monitors) cannot display all colors recorded.

Notes

Global Tonal/Color Correction

Reading the histogram -- many tools in photoshop asks you to deal with images through numbers.

Image -> Adjustments -> Levels (or Apple L)

Exercise:

1. in the histogram tool, move the grey triangle (called midpoint gamma) to one edge to make it darker or lighter.
2. click OK.
3. re-open the histogram tool and see what happenend to the histogram. Do you see a comb pattern?

Notes

Rounding Off

Notes

Additive color mode

- RGB: Red, Green, Blue
- When dealing with light

Notes

Subtractive color mode

- CMY(K): Cyan, Magenta, Yellow, blacK
- When dealing with pigments

Notes

Color mode

- Guess which color mode Photoshop uses?

- play around with color picker
- channels
- Image -> Mode ->