getting pixel data from image using libpng

mamacken · 01-11-2010, 10:34 AM

I am trying for create a program that performs 2-dimensional fast fourier transforms on each pixel of data in an image. I am going to transform each channel seperately, then map the coefficients to colours and recombine them into an image.

But I cannot seem to figure out how to read/interpret the pixel data using libpng. Can someone help me find out how I would read a png image and read each pixel's data into an array?

Adak · 01-11-2010, 10:51 AM

We don't work quite that way.

YOU show US what you've done to try to solve the problem, and where you're stuck - with code, preferably.

When we see code that you're working with, we can be specific. Without it, we can't be specific.

Is your file a PNG file? Did you read up on it's format? Did you look at it's contents with a Hex Editor? How did you open the file? What kind of variables did you use?

See what I mean?

mamacken · 01-11-2010, 11:18 AM

I don't really have code to show you that would be of any help. But I can be more specific with the nature of my problem.

My program reads in a PNG file and I can fetch it's width and height from the info structure. I also know how to read each row into an array of type png_bytep. I have no idea how I am supposed to access the individual color components of each pixel from the byte array though. I apologize for my vague description initially, I am just new to this particular library, and it's common usage is apparently quite different than what I am attemping to use it for.

brewbuck · 01-11-2010, 11:27 AM

If the pixels are 24-bit RGB, then the data is composed of interleaved and repeating R, G, and B samples. R comes first, then G, then B.

Code:

unsigned char *red = (unsigned char *)ptrToRow[ Y ] + 3 * X;
unsigned char *green = (unsigned char *)ptrToRow[ Y ] + 3 * X + 1;
unsigned char *blue = (unsigned char *)ptrToRow[ Y ] + 3 * X + 2;

mamacken · 01-11-2010, 11:37 AM

Thank you very much.

rogster001 · 01-11-2010, 11:59 AM

this is a good article, though it is SDL >

Lazy Foo' Productions

01-11-2010, 10:34 AM	#1
mamacken Registered User Join Date: Jan 2010 Posts: 3	getting pixel data from image using libpng I am trying for create a program that performs 2-dimensional fast fourier transforms on each pixel of data in an image. I am going to transform each channel seperately, then map the coefficients to colours and recombine them into an image. But I cannot seem to figure out how to read/interpret the pixel data using libpng. Can someone help me find out how I would read a png image and read each pixel's data into an array?
mamacken is offline

01-11-2010, 10:51 AM	#2
Adak Registered User Join Date: Sep 2006 Posts: 3,148	We don't work quite that way. YOU show US what you've done to try to solve the problem, and where you're stuck - with code, preferably. When we see code that you're working with, we can be specific. Without it, we can't be specific. Is your file a PNG file? Did you read up on it's format? Did you look at it's contents with a Hex Editor? How did you open the file? What kind of variables did you use? See what I mean?
Adak is offline

01-11-2010, 11:18 AM	#3
mamacken Registered User Join Date: Jan 2010 Posts: 3	I don't really have code to show you that would be of any help. But I can be more specific with the nature of my problem. My program reads in a PNG file and I can fetch it's width and height from the info structure. I also know how to read each row into an array of type png_bytep. I have no idea how I am supposed to access the individual color components of each pixel from the byte array though. I apologize for my vague description initially, I am just new to this particular library, and it's common usage is apparently quite different than what I am attemping to use it for.
mamacken is offline

01-11-2010, 11:27 AM	#4
brewbuck Senior software engineer Join Date: Mar 2007 Location: Portland, OR Posts: 5,768	If the pixels are 24-bit RGB, then the data is composed of interleaved and repeating R, G, and B samples. R comes first, then G, then B. Code: unsigned char red = (unsigned char )ptrToRow[ Y ] + 3 * X; unsigned char green = (unsigned char )ptrToRow[ Y ] + 3 * X + 1; unsigned char blue = (unsigned char )ptrToRow[ Y ] + 3 * X + 2; __________________ "Congratulations on your purchase. To begin using your quantum computer, set the power switch to both off and on simultaneously." -- raftpeople@slashdot
brewbuck is offline

01-11-2010, 11:37 AM	#5
mamacken Registered User Join Date: Jan 2010 Posts: 3	Thank you very much.
mamacken is offline

01-11-2010, 11:59 AM	#6
rogster001 Registered User Join Date: Aug 2006 Location: Liverpool UK Posts: 452	this is a good article, though it is SDL > Lazy Foo' Productions
rogster001 is offline

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