Contents

1. Types
2. Pointers/Arrays
3. Functions
4. Preprocessor
5. Compilation
6. Dynamic library

• Exercise 1: Design
• Exercise 2: Using pointers
• Exercise 3: Image processing example

Prerequisite: Basic knowledge of C

Reference: The C Language (ANSI C), Brian W. Kernighan & Denis M. Ritchie

Variables = {Type, Identificator, Value}

Scalar types

See the file <limits.h> and <float.h> to know the limitations.

Conversion

• Explicit: Casting example(double)var;
• Implicit: Rules of C. Be careful ! Prefer always explicit conversion.

Structure

Data composed as a set of variables

struct TImage {
	float *Data;
	long Nx, Ny;	
};

How to access to a field? . or ->

	struct TImage s;	/* structure */
	struct TImage *ps;	/* pointer to structure */

		s.Nx = 10L;
		ps->nx = 10L;

Enum

Type given a list of constants

		enum TOpenDay {Monday, Tuesday, Wednesday, Thursday};
		enum TOpenDay Today;
		Today = Tuesday;
	

Declaration of a pointer short *ps; short *ps = (short *)NULL; Unary operators & provide the address of an object: p = &a; * provide the content of an address: a = *p;

Access to pointed data

Using index notation

long Summation(long *p, long n) { long i, sum=0L; for(i=0L; i<n; i++) { sum += p[i]; } return(sum); }

Scanning from 0 to n-1

The addresses are numerical values: arithmetic operations are possible

long Summation(long *p, long n) { long i, sum=0L; for(i=0L; i<n; i++) { sum += *p++; } return(sum); }

Where is p now ?

Question: What is the difference between *(p++); and (*p)++; ?

Dynamic allocation

malloc: reserved a block of memory

free: release the reserved block

long *p; p = (long *)malloc((size_t)(1000L*(long)sizeof(long))); if ( p == (long *)NULL) { return (ERROR); } ... free(p);

Don't forget To allocate enough memory for your data. To check if there's available memory. To release the memory after usage.

Multidimensionnal arrays

The array data are organized follow as a raster:

How to access to a points (i,j,k) in volume of dimension [nx,ny,nz]

Value = Volume[i + nx*(j+ny*k)];

Modularity

Create a function when the inputs and outputs are well defined.

Passing parameters

• By value: The function works with a copy of the parameters: the parameters aren't modified by the function.

double BSpline( double x, long Order)

• By address: Give the pointer to the function: by this way the function can modify the parameters.

void Sobel( double *input, double *output)

Return

Return nothing:

			void Activate()
			{ 
				...
				return; 
				...
			}

Return result:

			long GetLength()
			{ 	
				...
				return(d);
				...
			}

Scope: extern vs static

Good practice to explicity qualify what it is the scope of a function.

Directive #include

Causes a copy of a specific files in place of the directive

2 forms:

• #include <stdio.h> < > for the C standard libraries
• #include "myfile.h" " " for the user headers

Directive #define, #undef

Usage 1: Create symbolic constants or macro (prefer functions instead macros)

		#undef PI
		#define PI ((double)3.14159265358979323846264338327950288419716939937510)
		#ifdef, #ifndef

Usage 2: Allow conditional compilation

		#undef DEBUG
		#define DEBUG
		...
		#ifdef DEBUG	
		printf("%d", a);
		#endif

5. Compilation

Compilation phases

Settings of your compiler

• Include files: Announce where are the include files
• Optimization: Good compromize among Speed / Small Code / Easy to debug
• Warning: Choose what are the level of warnings

6. Dynamic library

The compilation can produce an Application or Dynamic library

	Application	Dynamic library
	Stand-alone	Run inside an application
Loading	At the launch	When the calling application use the library
Entry point	1 entry: main()	No main() function The list of the callable functions are specified
Standard I/O	Usage of the console I/O: printf(),…	Don’t use the standard I/O. You don't know which application use your dynamic library
Windows	Executable/Application	Dynamic link library (.dll)
Macintosh	Application	Shared Library (.shlb)
Unix	Executable	Dynamic library (.so)

Execise 1: Design

Design a program for interactive exploration of 3D volumes of medical data. Identify user needs, propose a user interface. The design should include a short description of user interaction, short description of the internals, description of all functions and data structures.

Exercise 2: Using pointers

We would like to create a button from an image.

Source image

Result image

The size of the border is a parameter.

There are two basic operations:

• Rescale the gray level of the input image from 0 to 255.
• Add border around the image, dark gray in the south and east sides and light gray in the north and west sides. On the Macintosh, you find an empty framework file createbutton.c in the bilib:student that you can complete.

The interface to IDL is already done. So launch IDL, run init and the choose the dynamic library student.shlb. Then you can run for example:

IDL> show, createbuttonnowidget(lena(), border=10)

Exercise 3: Image processing example

We would like to write a function that does a smoothing filter using the moving average algoritm in a image (2D array) with mirror boundary conditions. The length of the moving window is a parameter. On the Macintosh, you find an empty framework file movingaverage.c in the bilib:student where that you can complete. The interface to IDL is already done. So launch IDL, run init and the choose the dynamic library student.shlb. Then you can run for example:

IDL> show, movingaveragenowidget(lena(), length=10)

Optional: extend this filter for a volume (3D array).