The document discusses image segmentation and the use of segments to structure image display. It describes how a display file can be divided into segments using a segment table. The segment table either uses arrays or linked lists to store segment information like start position, size, and attributes. Algorithms are provided for creating, closing, deleting, and renaming segments to dynamically manage the image display. Visibility attributes allow hiding or showing segments as needed.

1. Unit III
SEgments
Prepared By
Rajani Thite

2. Introduction
• To humans, an image is not just a random collection of pixels; it is a meaningful
arrangement of regions and objects.
• There also exits a variety of images: natural scenes, paintings, etc. Despite the large
variations of these images, humans have no problem to interpret them.
• The image information is stored in Display file.
• Existing structure of display file does not satisfy the requirements of viewing image.
Display image is modified to reflect the sub picture structure. To achieve this display
file is divided into segments.

3. The Segment table indicates the portion of the display file used to construct the picture
Display File
Segment Table
Display

4. Segment table

5.  The segment table is formed by using arrays. An alternative approach is linked list
 In case of arrays, maximum no. of segments that can be included in the segment
table are equal to the length of the arrays. But with linked list there is no such limit on
the maximum no. of segments; as list is dynamic
 In linked List ordering is achieved by simply adjusting the links. In case of arrays we
have to move actual segment information in the process of sorting the segments.
Segment Start Segment Size Scale X Scale Y Color Link
3
4
2
5
Null/
Segment Number
1
2
For ordering the segment
3
 The Disadvantage of linked list is that it requires more storage to hold the links
and that it is costly to locate arbitrary cells.
 In the linked list deleting the cell means changing two links.

6. Display file Linked List
OP X Y Link
1
2 2 .1 .5 5
3 1 .1 .1 2
4
5 2 .5 .5 9
6
7 2 .1 .5 0
8
9 2 .7 .5 7
10
Start=3

7. Segment Creation
 We Must give the segment name so that we can identify it.
 For example say that there is segment no. 3, then all following MOVE and
Line commands would belong to segment 3. We could then close segment 3
and open another.
 First thing to create a segment is to check whether some other segment is
open. We can not open two segments at the same time because we would
not know to which segment we should assign the drawing instructions. If
there is segment open, we have an error
 Next we should select valid segment name and check whether there
already exists a segment under this name. If so, we again have an error.
 The first instruction belonging to this segment will be located at the next
free storage area in the display file. The current size of the segment is zero
since we have not actually entered any instructions into it yet.

8. CREATE_SEGMENT(SEGMENT-NAME)
Argument SEGMENT-NAME
Global NOW-OPEN the name of currently open segment
FREE the index of the next free display-file cell
SEGMENT-START, SEGMENT-SEZE,VISIBILITY
ANGLE, SCALE-X, SCALE-Y, TRANSLATE-X,TRANSLATE-Y
Constant NUMBER-OF-SEGMENTS
BEGIN
IF NOW-OPEN>0 THEN RETURN ERROR ‘SEGMENT STILL OPEN’;
IF SEGMENT-NAME < 1 OR SEGMENT-NAME > NUMBER-OF-SEGMENTS
THEN RETURN ERROR ‘INVALID SEGMENT NAME’;
IF SEGMENT-SIZE[SEGMENT-NAME]>0 THEN
RETURN ERROR ‘SEGMENT ALREADY EXISTS’;
SEGMENT-START[SEGMENT-NAME]<-FREE;
SEGMENT-SIZE[SEGMENT-NAME]<-0;
VISIBILITY[SEGMENT-NAME] <-VISIBILITY[0];
ANGLE[SEGMENT-NAME] <-ANGLE[0];
SCALE-X[SEGMENT-NAME]<-SCALE-X[0];
SCALE-Y[SEGMENT-NAME]<-SCALE-Y[0];
TRANSLATE-X[SEGMENT-NAME]<-TRANSLATE-X[0];
TRANSLATE-Y[SEGMENT-NAME]<-TRANSLATE-Y[0];
NOW-OPEN<-SEGMENT-NAME;
RETURN;
END;

9. Closing a Segment
• Once the drawing instructions are completed, we should close it.
• NOW-OPEN variable indicates the name of the currently open segment.
• To close a segment it is necessary to change the name of the currently open
segment. It can be achieved by changing the name of currently open
segment as 0.
• We don’t have two unnamed segments around because we shall show only
one of them and the other would just waist storage.
• If there are two unnamed segments in the display file one has to be deleted.

10. Algorithm to close Segment
Global NOW-OPEN the name of currently open segment
FREE the index of the next free display-file cell
SEGMENT-START, SEGMENT-SIZE start and size of
the segment
BEGIN
IF NOW-OPEN=0 THEN RETURN ERROR ‘NO
SEGMENT IS OPEN’;
DELETE-SEGMENT[0];
SEGMENT-START[0]<-FREE;
SEGMENT-SIZE[0]<-0;
NOW-OPEN<-0;
RETURN;
END;

11. Deleting A Segment
 When we want to delete a particular segment from the display file then we
must recover the storage space occupied by its instructions and make this
space free for some other segment. To do this we must not destroy and re-
form the entire display file, but we must delete just one segment, while
preserving the rest of the display file.
 Here we have used arrays to store the display file information.
Segment 1
Segment 2
Segment 3
Segment 4
Unused Space
Segment 1
Segment 3
Segment 4
Unused Space

12. Delete Algorithm
1. Read the name of the segment which is to be deleted.
2. Check whether the segment name is valid; if not display error “Segment
not valid” go to step 8
3. Check whether the segment is open, if yes, display error message “ Can’t
delete open segment” go to step 8.
4. Check whether the size of segment is greater than 0, if no, no processing
is required as segment contains no instructions.
5. Shift the display file elements which follow the segment which is to be
deleted by it’s size.
6. Recover the deleted space by resetting the index of the next free
instruction.
7. Adjust the starting positions of the shifted segments by subtracting the
size of the deleted segment from it.
8. Stop.

13. Renaming a Segment
• The display processor is continuously reading the display file and showing its
contents
• Suppose we wish to use this device to show an animated character moving on the
display.
• To display a new image in the sequence we have to delete the current segment and
re-create it with the altered character. The problem in this process is that during the
time after the first image is deleted and time before the second image is completely
entered, only a partially completed character is displayed on the screen.
• We avoid this problem by keeping the next image ready in the display file before
deleting the current segment.
• Segment which is to be deleted and segment which is to be replaced with must exist
in display file at the same time.
• We do this by building the new invisible image under some temporary segment
name. When it is completed, we can delete the original image, make the replacement
image visible, and rename the new segment to become the old segment to achieve
apparent motion.
• The idea of maintaining two images, one to show and one to build or alter, is called
double buffering.

14. Algorithm to Rename Segment
1. Check whether both old and new segment names are valid; if not display error
message “Not valid segment names” and go to step 6.
2. Check whether any of the two segments are open. If open, display error message
“Segment still open” and go to step 6.
3. Check whether the new name we are going to give to the old segment is not
already existing in the display file. If yes, display error message “Segment
already exists” and go to step 6.
4. Copy the old segment table entry into new position.
5. Delete the old segment.
6. Stop.

15. Visibility
-Each Segment is given a visibility attribute.
-The segment’s visibility is stored in an array as part of the segment
table.
-By checking this array we can determine whether or not the segment
should be displayed.