Lab 4: Computer Graphics

Sun Oct 15 17:46:28 EDT 2006

By the end of this lab, you will be able to:

This lab will show you how to make your own pictures and buttons, logos and background images for your web page instead of mixing and matching the ones you've found on the Web. The lab assumes you're using PaintShop Pro, an elderly but simple image manipulation program that is available on OIT Windows cluster machines and downloadable, but you can use any program that provides similar capabilities. For example, both Windows and Mac cluster machines have a version of Adobe Photoshop, which is rather more complicated (and costs more if you buy it) but which has a lot more features.

Paintshop Pro is available on the cluster machines. If you are working at home, you can download version 4.1 and install it on your Windows machine. This version is ancient shareware; if you like it, buy the current version, which has a lot more features and costs a lot more.

If you use a program like Photoshop, you're on your own for details. We won't be very picky about what gets turned in, but do your best to submit what's asked for here.

Very Important: Material on other web sites may be subject to copyright, and there are both legal and University restrictions on what you can do with copyrighted material, including images and sound. The law is evolving in this area and copyright holders are becoming very aggressive in asserting their rights (and sometimes more than their rights). You should be aware of the University's policy on intellectual property rights.

In part, this says

"If there is an image, a background pattern, a section of text or a musical, film or video selection that you would like to use, you should make a good faith effort to determine that such use constitutes a "fair use" under copyright law or you must obtain permission of the owner or copyright-holder. As a general matter, you are free to establish links to Web pages you enjoy and which you would like to share with others. But you are not generally free to copy or redistribute the work of others on the World Wide Web (or elsewhere) without authorization and proper attribution."

Accordingly, you should be especially careful of images or sounds that might be of commercial value to their copyright owners. If a site has an explicit copyright notice, definitely do not copy anything from it.

For purposes of this lab, and in creating other web pages, you should add a citation for each image or sound that you link to or copy; this is equivalent to citing sources of quotations in a paper. If there is any ambiguity about whether a sound or image is copyrighted, use a link, not a local copy.

Part 1: How digital images work
Part 2: Creating your own images
Part 3: Using image filters
Part 4: Face pasting
Part 5: Creating image maps
Part 6: Submitting your work

Part 1: How Digital Images work

Pixels and Resolution

The computer's screen is divided into a large number of tiny dots of color called pixels ("picture elements"). These dots are so small that, to the human eye, they appear to make a continuous picture, but if you zoom in on any computer graphic, you will eventually begin to see the blocks of solid color of which the image is composed. To give you an idea of their size, the dot of an "i" is often represented by a single pixel.

A typical monitor or laptop screen can display at least 1024 pixels in the horizontal direction and 768 in the vertical, making almost 800,000 pixels. This figure is known as the resolution for monitors. Since a monitor has fixed dimensions, switching to a higher resolution causes it to display many more smaller pixels -- so you see more but everything is smaller. A laptop screen or flat panel display shows a fixed number of pixels, however, so changing the resolution may merely show you fewer pixels without changing the image size.

The same Desktop at three different resolutions
1024 x 768 800 x 600 640 x 480

Color Depth

Each pixel has its own color, typically represented by three values, one for red, one for green, and one for blue (RGB). These values control how much of that color appears in the pixel. Black would be all zeros, while white would have maximum values. Pure red would have just the red value raised to the maximum, while bluish-green might have the green and blue values set to half of the maximum, and the red value set to zero. A shade of gray has the same values for its red, green and blue components. All colors are represented by a combination of the three colors red, green, and blue.

In HTML, rather than use the name of the color as described in Lab 3, you can also use the RGB definition of the color in a manner similar to what is described above. Once again we start by using a tag to define the font color, and then using another tag to show where the color change ends. Instead of the name, we use "#rrggbb", where rr represents how much red we want, gg represents how much green and bb represents how much blue. The values are in hexadecimal, so each digit ranges from 0 to F. This means that the range for each 2-digit color is 00 to FF, or in decimal, 0 to 255. For example

<font color="#008888">This is Blue-Green</font>

does the following

This is Blue-Green

Your computer has video memory that holds the pixels that are displayed. For a fixed amount of video memory in the computer, there is a tradeoff between resolution and number of colors. For example, a display memory of 1 megabyte can have any of the following settings:

640 x 480 resolution 307200 pixels 3 bytes/pixel 224 = 16,777,216 colors
800 x 600 resolution 480000 pixels 2 bytes/pixel 216 = 65,536 colors
1024 x 768 resolution 786432 pixels 1 byte/pixel 28 = 256 colors

In the first case, each pixel requires 3 bytes of memory to store its color. In the second, 2 bytes are required and in the third, one byte suffices. But the third case involves more than twice as many pixels as the first.

The number of colors a display uses is known as its Color Depth. Color depth is also used to describe digital images. Unlike your screen, however, which has a fixed pixel-to-color relationship, images you create can usually be of any size and color depth. Ideally, you could always work with as many colors as possible, making photo-realistic images. Often, however, images which "use" 16 million colors will look the same when displayed in 256 colors. This is because they're not actually using more than 256 of the 16 million available colors.

Since large pictures take up more disk space and load more slowly on the Web, you may want to experiment with your images and see how few colors you get away with while preserving adequate image quality.

Image Formats and Compression

There are other ways to make image files smaller. Normally, images are saved as bitmaps, which use a few bytes for each pixel. This preserves the image perfectly but takes much more space than is necessary. Since most images have large blocks of constant color, we can use that information to group pixels and store less information about them. If there are 100 blue pixels in a row, it's more efficient to remember "100 blues" than to remember "blue, blue, blue, ..., blue, blue".

Taking advantage of pixel similarities to reduce the stored information is called compression. You will not need to worry about the specific techniques used to compress files. Instead, you can use two file formats which do this automatically.

To save your image as one of these types of files, choose GIF or JPG from the box underneath the Filename in the "Save As..." window.

Be sure that you set the file format to JPG or GIF before you save the file. It is not sufficient to use the extension .jpg or .gif in Paintshop. And do not use the .psd extension that is apparently the default in Photoshop.

There are many other image file formats. Perhaps the most common of these is TIFF, the Tagged Image File Format. TIFF is very widely supported by image manipulation programs like Photoshop and by devices like cameras and cameras. TIFF may provide lossless compression, but generally TIFF files are not compressed. Another increasingly common format is PNG (Portable Network Graphics), a newer format that combines some of the best features of GIF and JPG: it is lossless, but allows for many more colors.

Image Filters and Editors

To create and edit images, you must use an image editor like PhotoShop or PhotoShop. Generally, image editors are pretty self-explanatory, though there are a lot of menus with lots of sub-menus. They usually display one or more large canvases with a palette of tools. Choosing a paintbrush and dragging it onto the canvas works much like painting in real life. Choosing the paint bucket and clicking on an area will fill it with the current color. This lab is partly just a way to encourage you to experiment with a lot of these options.

Once you have drawn an image, you may wish to play with the Image Filters available. These can be found at the bottom of the Image menu in Paintshop Pro or on the Filter menu in Photoshop. These image filters (e.g., Sharpen, Emboss, etc.) will examine and possibly change each pixel in the image. The Sharpen filter, for example, looks for edges (places where the color changes dramatically) and enhances those.

Blurry image After Sharpen

Many filters only work on images with 16 million colors (24-bit color). This is usually because they make subtle color changes to the image and need to use many different shades.

Part 2: Creating your own images

Now it's time to experiment with some images. Again, this assumes Paintshop Pro; if you're using some other program, read this for the general idea, then find the equivalent mechanisms in your program.

You should see a large window with a few toolbars across the top and an empty gray area in the center. This center area is where your images will appear when you edit or create them.

To the left, you should see the Zoom Level control. You will be able to use this to zoom in and out of your image. On the right, there is a color selector. By clicking on the color you wish to use in the palette, you can cause it to be the foreground color. This is the color that will be used when you draw with the paintbrush. By right-clicking, you can adjust the background color, which is the color used for fills and some other changes.

You may also notice a "Histogram" window. You will not need this right now, so feel free to close it by clicking on its "X".

  1. Select "New" from the file Menu to create a new Image
  2. In the window that appears, make the width and height larger (at least 600 x 400)
  3. Choose a background color
  4. Change the Image type to "16.7 Million Colors"
  5. Press "OK"

A large window should appear, filled with the background color you chose. If your Paintshop window was not large enough for it, it may have automatically zoomed out. Be sure the Zoom Level is set to "1:1" or 100%. This large area is your canvas. Using the different tools available, you can paint on it with the mouse.

If you are unsure what a particular tool does, hold the mouse over its button and look at the description given in the bottom left corner of the window. If you make mistakes, just choose "Undo" from the Edit menu to undo your last stroke or action.

Shrinking, Enlarging, Cropping

To shrink or enlarge an image, choose "Resize" from the Image menu, and select Custom Size and type in a width and height. This should change your picture to a new size. If you clicked on the checkbox next to "Maintain aspect ratio", then the picture should look larger or smaller but the same otherwise. If you didn't then it is likely that your picture got stretched or squeezed in one dimension.

Unlike resizing, cropping an image will make it smaller by cutting off the edges. Click on the "Selection" tool (the dotted-line box between the hand and the lasso). Click on a part of the image and drag to create a box. Select "Crop" from the Image menu. Notice that everything outside that box has been removed from your picture.

Finally, you can change the picture back to its original size without enlarging the part you have drawn. Choose "Enlarge Canvas" from the Image Menu, and choose a new size. This should increase your drawing area. The new space is filled with the current background color.

Saving images

When you are ready to save your image, use the following steps to save it as a GIF or JPG.
  1. Choose "Save As" from the File menu.
  2. Select a Folder in which to save your file.
  3. Type in a file name.
  4. Choose either "GIF" or "JPG" as the file format next to "Save as Type".
  5. Press "Save".

If you are saving your file as a JPG, it is very important that you check the current compression rate in the following way:

  1. Choose "Preferences | File Format Preferences" from the File menu.
  2. Click on the tab entitled "JIF/JPG"
  3. Be sure that the compression level listed is reasonable. It is a percentage between 1 and 100 and should normally be around 10. Anything over 50 will probably result in a very noticeable distortion.
  4. Press OK.

You don't need to do a high-quality image here, though you might try to do better than this effort:

Part 3: Using image filters

Using the Built-in Filters

To best experience the image filters, you will open and edit a real photo image now.

  1. Download a picture from your web page or other source.
  2. In your Paintshop window, choose "Open" from the File menu.
  3. Locate the picture you just downloaded and open it.

The image should show up in its own window within the Paintshop window. As before, adjust the Zoom Level to your preference. (If the control is not there, click on the magnifying glass to make it appear.) Notice that you can edit and resize this image in exactly the same way as you did with the other image you just created.

  1. Check the color depth by choosing "Image Information" from the View menu and looking at the figure next to "Max # of Colors".
  2. If the color depth is less than 16 Million Colors, choose "Increase Color Depth" from the Colors menu and change it.

Now we're ready to try the built-in image filters, which are on the Image menu, under "Special Effects", "Deformations", "Edge Filters", "Normal Filters" and "Special Filters" (or, in Photoshop, on the Filter menu).

Try at least 3 of the filters and save the results as different files using the "Save As" option. You will be submitting these images at the end of the lab.

Creating a custom image filter (optional)

If you're interested in how filters work, try creating your own.

  1. Select "User Defined Filters" from the Image menu.
  2. Click on the "Help" button and read about how image filters work.
  3. Choose "New".
  4. Using what you learned in the "Help" section, edit the matrix of values to some new pattern and then press OK.
  5. Click on "Apply" to try out your new filter on your image.
You can do something analogous in Photoshop with Filter / Other / Custom.

Some Examples

Try to figure out which filters have the following effects:

Part 4: Face Pasting

Ever wonder how the tabloids are able to get front-page photos of Elvis and the president shaking hands? It's called photo-retouching and one of its most entertaining applications is face-pasting. Using graphics tools, you can cut someone's head out of one picture and paste it onto to the body of someone else in another picture, or do other merging of picture components. Here's an example from a previous version of COS 109:

OriginalNew and improved

Editing isn't limited to faces; here is a faked image of someone about to have a very bad day.

Find an interesting picture on the Web and paste pictures of yourself and/or your friends and/or someone or something noteworthy into the picture. Blend your picture in with the background as well as you can; we'll be looking for a good job. Save the image for submission.

Be sure to save often. You will probably invest a lot of effort in this, so don't let some random problem destroy your work. At the end of the lab, you will need to submit this image.

Here are a few tips:

  1. Find a picture that has a person in a similar pose to the picture you will paste in.
  2. Resize your picture to the right size to paste into the new picture
  3. Use the Lasso tool to select the right region of your picture, and then copy it
  4. Paste it into the new picture and move it to where you would like it to be
  5. Use the Retouch tool (the pointing finger) to smudge the edges around your head
  7. Some students end up spending a great deal of time on this, and then don't have enough time for the next section of the lab, "Creating Image Maps". Be careful with your time.

Part 5: Creating Image Maps

What's an Image Map?

Image Maps are images included in web pages which have different clickable parts that link to different places. Try out the following image map:

Right-click on the above picture and choose "View Image". You will see that the whole thing is one picture (even the buttons). Basically, to create a picture, you must create an image with a bunch of different rectangular parts that look like they should lead to other pages. Then, inside your HTML file, you can specify which areas of the picture link to which pages.

Before you start creating your own image map, look for a few examples on the Web. Any time you can click on different parts of a single image in order to go to different pages, that is an image map. The web sites of large corporations often have image maps.

Creating the Image

The first step in constructing an image map is to create the image that will be used for it. The image should have at least four rectangular areas that users can click on to go to different pages. If you'd like, you can create an image from scratch in Paintshop Pro. Otherwise, you may want to create a collage of photographs or graphics that you have found on the Web, like this:

For our example below, we chose one picture and then created rectangular sections within it. Review this example and then

Create your own image with several rectangular pictures or areas which could lead to different links. Save it to your public_html directory as a GIF or JPG file.

Do not make a picture that looks exactly like this one. Design your own image map!

  1. First, we found a picture:

  2. Next, we selected a section with the box-selector tool and blurred it:

  3. Then, we started making button-like areas by selecting a rectangular area and using the "Buttonize" filter:

  4. We created a total of 5 buttons:

  5. Finally, we added text with the text tool:

Again, you could use analogous facilities from Photoshop.

Getting the Coordinates

To make your image map work, you need to know the pixel coordinates (x, y) of the top left corner and the bottom right corner of each box that you wish to make into a link. Below, we've shown a few of the values for the rectangles in our example:

To find the coordinates of the boxes in your image, do the following:

  1. Select the "Selection Box" Tool (the dotted-line box).
  2. Without clicking, move it over your image and notice the numbers in the bottom left of your Paintshop window. These are the (x, y) coordinates of each pixel you move over.
  3. Find and record the coordinates of the upper left and bottom right corners of each box in your image. It doesn't matter if your values are a few pixels off!

This is pretty different in Photoshop. If you select Window and then Info, this brings up a small window that shows current coordinates, but by default in inches. If you then select Palette Options / Mouse Coordinates, you can convert that to pixels, which is what you want for an image mask.

For reference, here are the pixel values for OUR example:

  1. (0, 0) (128, 53)
  2. (0, 53) (128, 106)
  3. (0, 106) (128, 159)
  4. (0, 159) (128, 212)
  5. (0, 212) (128, 265)
Notice that y coordinates increas towards the bottom of the image.

Writing the html

Once you've created the image and found the coordinates for each box, you need to put all of that information in your HTML file. Look at our example and then follow the instructions to write the HTML code for your the image map you are creating.

Our example

For our example, we used the following HTML code:

<IMG SRC="im6.jpg" USEMAP="#imagemap">
<MAP NAME="imagemap">
<AREA SHAPE="rect" COORDS = "0, 0, 128, 53" 
<AREA SHAPE="rect" COORDS = "0, 53, 128, 106" 
<AREA SHAPE="rect" COORDS = "0, 106, 128, 159" 
<AREA SHAPE="rect" COORDS = "0, 159, 128, 212" 
<AREA SHAPE="rect" COORDS = "0, 212, 128, 265" 

First, we include the image with the IMG tag just like any other image, except that we tell the browser where to find its map (with the attribute USEMAP). Then we use the MAP tag to start a map section called "imagemap". Next, for each box, we include an AREA tag with the shape, coordinates and appropriate link. Finally, we close the MAP tag with </MAP>

What you should do

  1. At the place in your HTML file where you would like your Image Map to be, type the following line:
    	<IMG SRC="myimage.jpg" USEMAP="#MyImageMap">
  2. Replace "myimage.jpg" with your image's filename.

This will display your image just like any other image you have included in your web page. The USEMAP attribute, however, will let the browser know that this image is an image map and will tell it where to find the information it needs to make all of the links inside it work properly.

Once you have placed the image, you need to include a section somewhere within the HTML file that contains that mapping information. Copy the following lines to your HTML file:

	<MAP NAME="MyImageMap">
	<AREA SHAPE="rect" COORDS="x1, y1, x2, y2" HREF="link.html">

The first of these lines tells the browser that everything it sees until the </MAP> tag will be information about the links in an image map named "MyImageMap". The second line gives link information for one rectangle in your picture. Follow these directions to fill in information for all of your rectangles.

  1. Make a copy of the second line (<AREA ...) and paste it after the first. Repeat this until you have one <AREA ... line for every box in your picture that you wish to be a link; there must be at least four.
  2. For each box in your picture, fill in the AREA information in the following way:

When you have filled in one AREA tag for each box in your picture, save your HTML file and open it in your browser to try out your image map. You should now have a working image map (which you will need to submit). If you are ambitious, you may want to create a more interesting image map with different shapes.

It is important to realize the difference between image maps and picture links. A picture link is simply a link that has a picture rather than text as the "clickable" region. With an image map, more can be done -- one can divide the image into sections, and treat each section separately. This cannot be done with a picture link. The similarity is that in each case, a "button" is nothing more than a picture, until the HTML tags are added to make it a link.

Part 6: Submitting your work

Place all of your work online in a file called lab4.html and just submit the URL of that page.

  • lab4.html must display at least your original image and the 3 filtered versions, your face-pasted image, and your image map.
  • Save lab4.html in your public_html directory on Arizona;
  • Make an obvious link from your home page (index.html) to lab4.html.
  • Make sure everything works and is accessible; get a friend to play dumb and try it all.
  • Please do check all the links and all the buttons. If we can't read it, we can't grade it, and you can't get credit for it. Thanks.

    Send an email to with the URL ( of that file. If you saved anything on the Desktop, be sure to transfer it to your public_html folder on Arizona.