can not read png file with libpng

问题

I am new to libpng and the documentation is really confusing for me. Below is my code which is not working and I do not see the reason why. Can someone point me to right direction? or suggest different ( "easier" ) library?

how I understand libpng:

open the file with fopen in rb mode
create png_structp with png_create_read_struct
create png_infop with png_create_info_struct
allocate space

read data

#include <stdio.h>
#include <png.h>

int main( int argc, char **argv )
{
    int x, y;
    int height, width;

    png_structp png_ptr;
    png_infop info_ptr;

    png_bytep *row_pointers;

    FILE *fp = fopen( "test.png", "rb");
    {
        if (!fp)
            printf("File could not be opened for reading");

        png_ptr = png_create_read_struct(PNG_LIBPNG_VER_STRING, NULL, NULL, NULL);

        info_ptr = png_create_info_struct(png_ptr);

        png_read_info(png_ptr, info_ptr);
        width = png_get_image_width(png_ptr, info_ptr);
        height = png_get_image_height(png_ptr, info_ptr);

        row_pointers = (png_bytep*) malloc(sizeof(png_bytep) * height);
        for (y=0; y<height; y++)
            row_pointers[y] = (png_byte*)malloc(png_get_rowbytes(png_ptr,info_ptr));

        png_read_image(png_ptr, row_pointers);

        fclose(fp);
    }

    for (y=0; y<height; y++) 
    {
        png_byte *row = row_pointers[y];
        for (x=0; x<width; x++) 
        {
            png_byte* ptr = &(row[x*4]);
            printf("Pixel at position [ %d - %d ] has RGBA values: %d - %d - %d - %d\n", x, y, ptr[0], ptr[1], ptr[2], ptr[3]);
        }
    }
}

回答1:

I similarly had failure in png_create_read_struct. It is difficult to debug without further info (in my case, stderr goes nowhere), but fortunately you can provide your own error and warning functions:

void user_error_fn(png_structp png_ptr, png_const_charp error_msg);
void user_warning_fn(png_structp png_ptr, png_const_charp warning_msg);

Using this, libpng printed helpful errors stating that the application was using a different version of the header than the libpng found at runtime. Mystery solved.

So while individual issues may differ, using libpng's error reporting should provide insight.

回答2:

Obviously, there is no information passed to libpng about where/how to read image chunk. Use:

... png_init_io(png_ptr, fp); png_read_info..

来源：https://stackoverflow.com/questions/10011925/can-not-read-png-file-with-libpng

标签

libpng