1.1   Opening a raster image

Bilko supports a number of different image formats. You will start by opening one of the simplest, a basic 2-D grid of 8-bit data (pixel values 0-255), an ATSR image of sea surface temperature (SST) called ats199701.gif

Note: If you have not yet downloaded the images you need for this section of the tutorial, you should do so now. The links on the left sidebar leads to zip files containing the images you will need.

There are several ways to open a file; any one of the following will do the trick:

From the menu bar:
  - either click on File, then Open on the drop down menu,
  - or hold down the [ALT] key and type [F], then type [O].
Toolbar:
  Click on the Open File Icon - .
Keyboard shortcut:
  Hold down [CTRL] and type [O]. (Shorthand: [CTRL+C] ).
 

The 'Open' dialog

The 'Open' dialog ( figure (9K)) in Bilko works in a similar way to those in other Windows applications. It allows you to

• select the folder to look in
• select a file from a list of files in this folder by clicking on it (once *), which will then appear in the 'File name' box, and
• select the File type to be listed in this folder - the default is IMAGES,
  

* Note that if you double click the file will open immediately and you do not get a chance to check that the settings of the dialog are what you want.

At the bottom of the dialog you will see three little check boxes that are uique to Bilko:

• The Extract box,
• The Apply box, and the
• The Minimize box.

You can use Bilko Help to find out what these boxes do. Place your mouse pointer over the text next to each box in turn and press [F1]. Then answer the questions below.

Question 1.
a)	Two of these boxes apply only to images. Which are they?
b)	What happens to image file you open if the 'Minimize' box is checked?

To open the first image file you simply:

1. Leave the file type at the default option, IMAGES. (You will encounter the alternatives in later lessons.) As you can see 'IMAGES' includes a variety of file formats. By the end of the tutorial you will be familiar with how to open and examine all of these.
   Note: If you or someone else have recently been opening another type of file the 'File type' box may need changing before you will be able to see the list of images. If you cannot see the file you're after, check that this box is set correctly. You can change it using the drop-down on the right side of the box.

2. Select the file atsrsst199701.gif from the directory named tutorial_data.
   Note:This should be the default directory for the tutorial. You may have your images stored somewhere else, if so, use the drop down arrow next to the 'Look in' box to navigate to that directory.

3. Make sure the 'Extract' and 'Minimize' boxes are both unchecked

4. Click the OK button.

You should see a greyscale image of world sea surface temperature SST, measured by the ATSR2 infrared radiometer. (The sidebar has a link to more information about the images in this section).

The 'Extract' dialog

You will now open a second image, in a slightly different way, using the Extract dialog to sample the image.

1. Open the 'Open' dialog ([CTRL+O]) and select the file atsrsst199801.gif from the list of files.
2. Make sure the Extract box is checked
3. Click the OK button.

The Extract dialog will appear ( figure (5K)).

The title bare (blue bar at the top) of the'Extract' dialog tells you how big the image is. In this case the size is 720 x 360 pixels; that is there are 720 pixels in the X-dimension (i.e. 720 columns) and and 360 pixels in the Y-dimension (i.e. 360 rows). For more information about the structure of image data see Image data: A grid of pixels.

The Extract dialog allows you to decide how you want to load this image:

• You can set size of the Window to be opened by setting the first and last column (X) and the first and last row(Y) to load.
• You can also set the Sample rate: 1 opens every pixel, 2 opens every other pixel, 3 opens one pixel in three, etc.

Note: As a default the Extract box is set to open the whole image at full resolution. You may want to change the default if you usually deal with very large images, but in general the standard Bilko default works well for all the images in this tutorial, so there is no need to do this now.