CSE 803 Using images in C

1
CSE 803 Using images in C

• Computing with images
• Applications Methods
• 2D arrays in C
• Programming project 4

2
Memory organizations

• Raster storage of 2D array
• 2D array using pointers to rows
• PGM image file format

3
1D array via a pointer

• P is base address of array in memory
• Array has N cells
• Cells 0, 1, , N-1
• double P
• P new doubleN
• Pk located at
• P k size of cell

4
2D array via pointers

• int P
• Need to acquire dynamic memory for row pointers
  and for rows of integers

5
Point to array of row pointers

• typedef int integerPointer
• integerPointer P
• P new integerPointer 5
• // allocate 3 columns in each row
• for ( int k0 klt5 k )
• Pk new int 3
• // now have 5 rows of 3 integers each

6
Referencing 5 rows of 3 ints

• // put i j in Pij
• for ( int i0 ilt5 i)
• for ( int j0 jlt3 j)
• (Pi)j ij // dont need ()
• // P points to a 1D array of pointers to int
• // Pi is the pointer offset i from P
• // (Pi)j is the int offset j from PI

7
raster storage of 2D array

• One continguous block of memory for double
  A34
• 3 rows of 4 columns
• 8 bytes for each cell
• Row 0 first, then row 1, then row 2
• Easy formula locates any cell in same time

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Issues with 2D arrays

• Images can be different sizes R x C
• Size unknown until image file opened
• Image history usually needed
• Program must keep collection of data items R,
  C, comments, Irc
• Object class is a good solution

9
PGM is simple image file form

• See Fig. 2.12 of Shapiro and Stockman
• P2 is magic number for gray image
• indicates comment line
• Width, height, max gray value next
• ASCII values for width x height pixels in row
  major order

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PGM image with ASCII info.

• P2 means ASCII gray
• Comments
• W16 H8
• 192 is max intensity
• Can be made with editor