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Question

我正在尝试使用 libpng 读取 PNG 数据，但我无法弄清楚最后一步

你好， 所以在过去的两天里我试图读取 png 像素数据。我需要这方面的帮助。这是我想到的

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

#define BYTES_TO_CHECK 8

// checks the file and returns the file pointer
FILE *check_png(char *file_name) {
    char buffer[BYTES_TO_CHECK];
    FILE *fP = fopen(file_name, "rb");
    if (fP == NULL) {
        printf("FILE NOT FOUND!");
        exit(0);
    }
    fread(buffer, 1, BYTES_TO_CHECK, fP);
    int check = png_sig_cmp(buffer, 0, BYTES_TO_CHECK);
    if (check == 1) {
        printf("NOT A PNG");
        exit(0);
    }
    return fP;
}


int main() {

    png_structp png_ptr;
    png_infop info_ptr;
    png_uint_32 height;
    png_uint_32 width;

    printf("ASCII Generator\nPlease Enter the PNG file name\t");
    char file_name[100] = "google.png";
    /*scanf("%s", file_name);*/
    FILE *fP = check_png(file_name);

    png_ptr = png_create_read_struct("1.6.44", NULL, NULL, NULL);
    info_ptr = png_create_info_struct(png_ptr);

    if (png_ptr == NULL)
        printf("ERROR PNG POINTER IS NULL");

    else if (info_ptr == NULL)
        printf("ERROR INFO POINTER IS NULL");

    png_init_io(png_ptr, fP); // initialising input output
    png_set_sig_bytes(png_ptr, BYTES_TO_CHECK); // tells that some bytes are already read

    png_read_png(png_ptr, info_ptr, PNG_TRANSFORM_IDENTITY, NULL);   // reads png

    height = png_get_image_height(png_ptr, info_ptr);
    width = png_get_image_width(png_ptr, info_ptr);
    printf("%d %d", height, width);

现在读取像素值时遇到一些问题

png_bytep row_pointer[height];
png_read_image(png_ptr, row_pointer);

返回

Read Error

并且中止 然后我就尝试了

png_bytepp row_pointer = png_get_rows(png_ptr, info_ptr);

它返回一个指向每行像素数据的指针数组，但它都是空的

for (int i = 0; i < height; i++) {
        printf("%s", row_pointer[i][50]);
    }

我也尝试过 png_read_rows 但无济于事。 请帮帮我，谢谢！ :)

Answer 1

首先，您传递了

info_ptr

却没有获得一些有意义的数据。您没有对函数返回进行错误检查。

您为什么不直接查看

libpng

的官方示例，并以此为基础编写您的代码？您显然缺少示例代码的某些部分。

摘自

libpng

的示例代码

                  png_uint_32 width, height;
                  int bit_depth, color_type, interlace_method,
                     compression_method, filter_method;
                  png_bytep row_tmp;

                  /* Now associate the recently opened (FILE*) with the default
                   * libpng initialization functions.  Sometimes libpng is
                   * compiled without stdio support (it can be difficult to do
                   * in some environments); in that case you will have to write
                   * your own read callback to read data from the (FILE*).
                   */
                  png_init_io(png_ptr, f);

                  /* And read the first part of the PNG file - the header and
                   * all the information up to the first pixel.
                   */
                  png_read_info(png_ptr, info_ptr);

                  /* This fills in enough information to tell us the width of
                   * each row in bytes, allocate the appropriate amount of
                   * space.  In this case png_malloc is used - it will not
                   * return if memory isn't available.
                   */
                  row = png_malloc(png_ptr, png_get_rowbytes(png_ptr,
                     info_ptr));

您可以访问官方存储库示例并以此为基础编写您的代码。