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Meaning The width parameter is valid and contains the width of the image The height parameter is valid and contains the height of the image The properties associated with the image can now be obtained using imgObjgetProperty( ) More pixels needed to draw the image have been received The parameters left, top, width, and height define the rectangle containing the new pixels A complete frame that is part of a multiframe image, which was previously drawn, has been received This frame can be displayed The left, top, width, and height parameters are not used The image is now complete The left, top, width, and height parameters are not used An error has occurred to an image that was being tracked asynchronously The image is incomplete and cannot be displayed No further image information will be received The ABORT flag will also be set to indicate that the image production was aborted An image that was being tracked asynchronously was aborted before it was complete However, if an error has not occurred, accessing any part of the image s data will restart the production of the image

A sample entry from the /etc/shadow file is shown here for the user account mmel:

TABLE 25-1

Here is a simple example of an imageUpdate( ) method:

mmel:$1$HEWdPIJ$qX/RbBTPGcyerAVDlF4g:12830:0:99999:7:::

public boolean imageUpdate(Image img, int flags, int x, int y, int w, int h) { if ((flags & ALLBITS) == 0) { Systemoutprintln("Still processing the image"); return true; } else { Systemoutprintln("Done processing the image"); return false; } }

This chapter covers the final block in the Common Palette. When you are finished with this chapter, you will have the ability to create some great programs for your robots and give them the ability to move, talk, listen, bump, stop, wait, and a lot more. (But there are plenty more NXT-G blocks to learn about, so don t stop reading yet!) Let s give your robots one more talent the ability to make choices and select from multiple possible actions. Choices are very important to your robot; with the ability to decide between two or more options, your robot can perform much more complicated actions.

Not only are images useful for storing pictures, as we ve just shown, but you can also use them as offscreen drawing surfaces This allows you to render any image, including text and graphics, to an offscreen buffer that you can display at a later time The advantage to doing this is that the image is seen only when it is complete Drawing a complicated image could take several milliseconds or more, which can be seen by the user as flashing or flickering

The /etc/group file contains a list of groups, with one group per line Each group entry in the file has four standard fields, with each field colon-delimited, as in the /etc/passwd and /etc/shadow files Each user on the system belongs to at least one group, that being the user s default group Users may then be assigned to additional groups if needed You will recall that the /etc/passwd file contains each user s default group ID (GID) This GID is mapped to the group s name and other members of the group in the /etc/group file The GID should be unique for each group Also, like the /etc/passwd file, the group file must be world-readable so that applications can test for associations between users and groups The fields of each line in the /etc/group file are Group name The name of the group Group password This is optional, but if set, it allows users who are not part of the group to join Group ID (GID) Group members The numerical equivalent of the group name A comma-separated list

Part II:

This flashing is distracting and causes the user to perceive your rendering as slower than it actually is Use of an offscreen image to reduce flicker is called double buffering, because the screen is considered a buffer for pixels, and the offscreen image is the second buffer, where you can prepare pixels for display Earlier in this chapter, you saw how to create a blank Image object Now you will see how to draw on that image rather than the screen As you recall from earlier chapters, you need a Graphics object in order to use any of Java s rendering methods Conveniently, the Graphics object that you can use to draw on an Image is available via the getGraphics( ) method Here is a code fragment that creates a new image, obtains its graphics context, and fills the entire image with red pixels:

A sample group entry in the /etc/group file is shown here:

Canvas c = new Canvas(); Image test = ccreateImage(200, 100); Graphics gc = testgetGraphics(); gcsetColor(Colorred); gcfillRect(0, 0, 200, 100);

Once you have constructed and filled an offscreen image, it will still not be visible To actually display the image, call drawImage( ) Here is an example that draws a timeconsuming image, to demonstrate the difference that double buffering can make in perceived drawing time: