1) What is a bitmap?
a) Bitmap or Raster images are composed of a grid of individual pixels.
b) Each pixel has one solid, specific color
c) The Bit Depth of the image determines how many different colors any
given pixel in the image can be.
i) So, in a black and white bitmap image, any pixel can be either
black or white- a bit depth of two.
d) In essence, bitmap images function like huge, intricate Pointilist
paintings- your eye sees the individual pixels as making up a gradient.
e) As a test, let’s take a very close look at a bitmap image. Go to the class blog, and click on the image at the top. This should bring up a window with only the image in it. Drag the image to your desktop, and then open it in Photoshop. (It’s called ResolutionDemo.jpg)
f)To see the individual pixels in this image, go to the View menu in Photoshop, then scroll down to Show, and over to Grid. Then, go to the Photoshop menu, to Preferences, then down to Guides and Grid.
g) Under Grid, select Gridlines Every 1 Pixel, and 1 Subdivision.
Hit OK.
Then, click on the Zoom tool, and zoom in to 800% or 1200% (these numbers show up on the bottom left of your image window). You’ll see individual boxes of flat color- each one of these represents one pixel in your image.
2) Resolution in Bitmap Images
a) If bitmap images are grids, resolution is determined by the number of
pixels in the grid.
b) We talk about resolution in terms of PPI (Points per Inch/Pixels per
Inch) or DPI (Dots per Inch) PPI is for on-screen images, and DPI
refers to printed images, or images we intend to print.
c) If you open an image in Photoshop, and then go to Image- Image Size,
a window will show you the size in inches and pixels of your image. The window also shows you the resolution of the image in Pixels per Inch.
d) Our test image here is 3” x 5”, at 100 DPI. If we multiply 3 inches by
100 PPI, this gives us a dimension of 300 pixels, which we see above in the Pixel Dimensions window. So, you can always determine the pixel dimensions of an image this way, by multiplying the PPI by the actual dimensions.
e) Images with higher resolution, or larger dimensions, hold more
information and take up more hard drive space.
f) The two most important numbers in terms of resolution are 72 dpi and
300 dpi. 72 dpi is the resolution of most computer monitors- so you
need at least 72 dpi at your finished size to look good on a screen. 300 dpi is print resolution- so you need at least 300 dpi at your finished size to look good printed. Because screen resolution is much lower than print resolution, an image that looks good on screen may not look good printed.
g) As an experiment, download an image off the Internet and open it in
Photoshop. Most images from the web are at 72dpi, and look good at their normal size. However, if you zoom in on the image in Photoshop, it will very quickly break apart into pixels and look ‘pixelated’. With an image at higher resolution, you can zoom in much further before it begins to break down.
3) Resizing Bitmap Images
a) The next step in understanding bitmap images is resizing an image.
b) The same ‘Image Size’ window in Photoshop not only tells you the
dimensions of your image, but also allows to you change its size.
c) Make sure you have ‘Constrain Proportions’ checked, and ‘Resample Image’ unchecked.
d) Now, change the resolution of the image to 50 dpi. The dimensions of the image in inches will double, though the pixel dimensions will not. So, as long as ‘Resample’ is unchecked, the amount of information in the image stays constant. You are simply stretching or compressing the same amount of information over a larger or smaller space. Think of it as each pixel doubling in size: you have the same total amount of pixels, but to make them cover twice the area, each one must be twice as large.
e) So, if you have an image from the Internet at 72dpi, if you make it
approximately four times smaller, it will approach print resolution of 300dpi. Conversely, you can make a print-resolution image much larger for uploading to the Internet. In either case, you are simply reconfiguring the existing image- you’re not adding or subtracting information.
f) The key concept here is that an image contains a fixed amount of
information- any changes to the amount of information require either discarding information (decreasing pixel dimensions) or adding information (increasing pixel dimensions).
h) As long as the pixel dimensions of an image stay the same, you aren’t actually changing the information in the image- you’re just distributing it differently. But, the thing you want to avoid is making the pixels so big that you can see them. You can increase the inch dimensions of your pictures as much as you’d like- but you’ll start seeing pixels fast. It’s even easier to spot pixels in a printed image- so you need more pixels to print than for monitor viewing.
4) Using the Resample option
a) Now, let’s check the Resample box. This is a very different type of
resizing than what we were doing with Resample off.
b) With Resample, you can change the amount of information in an image,
either increasing or decreasing it. If you change the resolution field now, the dimensions of the image stay the same.
c) What you need to remember here is that you can always downsample
(make smaller). This is simply a process of ‘throwing away’ extra information. Either changing size or resolution makes the image smaller. However, you should always save your smaller image with ‘Save As’, to make a copy and preserve your original hi-res file.
c) Upsampling. Photoshop allows you to increase the size or resolution of
the file. However, you do not gain much by doing this- the amount of information is fixed, and by asking Photoshop to increase the size, you are simply making Photoshop ‘guess’ or interpolate extra pixels. This usually gives a fairly poor result, especially with low-resolution images. So, this is why printing an image from the Internet, even if you’ve upsampled it, usually gives a pixelated result.
d) So, just because Photoshop tells you that you have a 300dpi image, it may still look bad printed- because you didn’t start with a good-quality image.
Bitmap File Types
1) JPEG,
a) abbreviated as .jpg
b) Stands for Joint Photographic Experts Group
c) Most commonly used for web display of images
d) Compressed format- converting a file from TIFF or PSD to JPEG involves discarding information.
e) Smaller files than TIFF or PSD images
f) Poor results when printing from JPEGs- avoid this if possible
g) compatible with a wide range of graphics programs
2) TIFF
a) abbreviated as .tif
b) stands for Tagged Image File Format
c) Generally uncompressed format
d) Good for output from scanners
e) Good for printing
f) Larger file size than JPEG- poor for web use
g) Compatible with a wide range of graphics programs
3) PSD
a) abbreviated as .psd
b) Stands for Photoshop Document
c) Proprietary file type, and default for Photoshop
d) Uncompressed file format
e) Best for printing from Photoshop
f) Allows full use of all Photoshop features
g) Largest of the file types due to the amount of information Photoshop includes with the file.
Intro to Scanning- Flatbed Scanner
1) Set up Scanner
a) Turn on scanner
b) Start Epson Scan from the Dock
c) Open top of scanner
d) Place your image, face down, on the scanner glass.
e) Many objects can be scanned- just so long as they
1) Do not scratch the scanner glass
2) Do not leave residue on the scanner glass
f) In Epson Scan, make sure you have ‘Reflective’ selected-
because you’re scanning an opaque object.
g) Hit Preview/Prescan at the bottom of the window
2) Set up your Scan
a) Once the scanner has generated a preview of your image,
select the area you’d like to scan (the scanner previews the entire
scanning bed initially)
b) Select resolution for your scan. If you plan on printing larger than your original image, you should scan at more than 300dpi- for example, scan at 600 dpi if you plan on doubling the size of the image.
c) Select your output format. For our purposes, select SAVE, and then select TIFF for the file format. This is an uncompressed format that can easily be converted to PSD for Photoshop use.
d) Try out enhancement options in the scanner window- especially Auto Color, which can significantly improve the color quality of your image.
e) Hit Scan. Epson Scan will ask you where to save the image, and what format you’d prefer to use- select TIFF, and pick a place to save it.
g) Find the file on your computer, and open it in Photoshop.
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