read part of png file

Good mornning everyone

I'm trying to read a part of a PNG image file using Libpng and VC++ 2008 but unfortunately while executing the program an error " access violation memory" occured

Code:

 // Initialisation
png_bytep row_pointers;
png_structp png_ptr ;
..................
.....................
BYTE * buffer = (BYTE *)malloc((size*sizeof(BYTE)); //size : is less than image lenght 
row_pointers = (png_bytep)(buffer +size);
 png_read_image (png_ptr, row_pointers); //the problem occured here
...............
..............

Please help me and thanks in advance

Did you read the manual?
http://www.libpng.org/pub/png/libpng-manual.txt

Here's what they have as an example.

Code:

   if (height > PNG_UINT_32_MAX/png_sizeof(png_byte))
      png_error (png_ptr,
          "Image is too tall to process in memory");

   if (width > PNG_UINT_32_MAX/pixel_size)
      png_error (png_ptr,
          "Image is too wide to process in memory");

   row_pointers = png_malloc(png_ptr,
       height*png_sizeof(png_bytep));

   for (int i=0; i<height, i++)
      row_pointers[i]=NULL;  /* security precaution */

   for (int i=0; i<height, i++)
      row_pointers[i]=png_malloc(png_ptr,
          width*pixel_size);

   png_set_rows(png_ptr, info_ptr, &row_pointers);

Your row_pointers are basically garbage.

Thank you Salem for your reply , but when I put

Code:

 row_pointers = png_malloc(png_ptr,       height*png_sizeof(png_bytep));

an error produce "Impossible to convert 'png_voidp' to 'png_byte **' "
and for

Code:

row_pointers[i]=png_malloc(png_ptr,          width*pixel_size);

"Impossible to convert 'png_voidp' to 'png_byte *'

I declare row_pointers as : png_byte ** row_pointers = NULL;

Could anyone help me
thanks in advance

Well that sounds like it's because you're using C++ to compile C code.

Code:

# compile as C
$ gcc bar.c -lpng

# compile as C++
$ gcc -x c++ bar.c -lpng
bar.c: In function ‘int main()’:
bar.c:8: error: invalid conversion from ‘void*’ to ‘png_byte*’

You could force the conversion with a cast, or perhaps just name your source files as say prog.c, if in fact you're intending to write C code.

Thank you so much Salem it works now

I think that this problem will drive me crazy ...... when I try to read the data from row_pointers, it seem to be completely different from the image data

png_set_rows (png_ptr, info_ptr, row_pointers);
for(int p = 0; p < row_Lenght; p++)
cout << row_pointers[0][p] << endl;

Where is the problem ???? Thanks again

Have you created (using your favourite image editing software) a small set of test PNG files which are say 10x10 all red pixels?

Saying "my data is junk" whilst neither posting your image, or your "garbage" results doesn't really help anyone.

First of all I'm sorry for taking this long time to put my comment .... Your are right Salem, after checking the first 10 byte of my whole image (To do the check I red all the image using png_read_image " (png_ptr, row_pointers) " the result was "13, 13, 13, 13, 13, 13, 13, 13, 13, 14 " but when I used "png_set_rows ()" to read only part of the image, the first 10 byte was "205, 205, 205, 205, 205, 205, 205, 205, 205, 205" and actually all the byte have this same value ... I think that when I used " png_set_rows () " it don't read the value of the image. So maybe I need to add a function to do it ???
I really appreciate your help

PS : I'm not using png_read_image() because an access violation error appear

I've just used the function " png_read_rows(png_ptr, row_pointers, NULL, 1) ; " but the result for the first 10 byte was " 24, 25, 25, 25, 25, 25, 26, 26, 26, 26 " ......