How do you turn a .png file into a 2-d matrix?

Question

I am working on a project for a Bio-medical Imaging course (Note: it is a non-programming course, so asking for programming help is not cheating. Asking for conceptual help with planning would be cheating.) where I need to manipulate an image using different mathematical transforms. I am writing in C so it can be as fast as possible. I have finished the code for the mathematical transforms, but I have realized that I do not know how to turn a grayscale .png file into a 2-d matrix/array to compute with, and I do not know how to display a .png file in C. Can anyone help me?

I'm trying to turn the "image.png" image into a 2d array where each entry in the array has a value between 0 - 255 and corresponds with each pixel in "image.png". I also want to turn a 2d array where each entry corresponds to a pixel in the image and has a value between 0 - 255 into a new "image_two.png" file.

I'm a somewhat new programmer. I have a solid base in python programming, but C is new for me. I have done a lot of research and I have found a lot of people talking about using "this library" or "that library", or also "this library", but how do I use a downloaded library in C? It's unfamiliar territory for me as a python programmer :(

I'm using Ubuntu 12.04

To reiterate:

• How do you read a grayscale .png image as a 2-d array/matrix in C?

• How do you display a 2-d array/matrix as a grayscale image in C?

• How do you use a downloaded library in C code (specifically for the two questions above)? I found out how to use these libraries.

EDIT: I am still having trouble figuring out how to create a grayscale 2d array out of a .png file and how to make a .png file out of a grayscale 2d matrix. Can anyone else help?

Answer 1

You can use a more general purpose image handling library and you might find it easier to use. I recommend FreeImage http://freeimage.sourceforge.net/. See the pixel access section of the manual to get access to the pixel data. You can then work with it directly or copy it into your own matrix.

Answer 2

To install a library in Linux, typically you will use the package manager. For example, in Debian (this includes Ubuntu) you might do:

$ apt-cache search libpng

You'll decide which package to install based on the results of running this command and then you will run

$ sudo apt-get install <package-name>

This command will likely install png.h in a location that is already included in gcc's search path. This means that to use png.h in your program, all you have to do is include it.

#include <png.h>

Skip to chapter 3 in the libpng manual for a walkthrough on reading a png file.