PNG 文件被截断并且不能被每个应用程序显示

Question

我有一个生成 PNG 文件的代码（使用 libpng）。我可以使用 EOG (Eye Of Gnome) 打开这些文件，但使用 GIMP、imagemagic 和其他工具时我遇到了错误。 Exiftool 告诉我 png 文件被截断了，但我看不到在哪里。在 EOG 上一切正常。

代码：

int savepng(const char *name, fits *fit, uint32_t bytes_per_sample,
        gboolean is_colour) {
    int32_t ret = -1;
    png_structp png_ptr;
    png_infop info_ptr;
    const uint32_t width = fit->rx;
    const uint32_t height = fit->ry;

    char *filename = strdup(name);
    if (!ends_with(filename, ".png")) {
        filename = str_append(&filename, ".png");
    }

    FILE *p_png_file = g_fopen(name, "wb");
    if (p_png_file == NULL) {
        return ret;
    }

    /* Create and initialize the png_struct with the desired error handler
     * functions.  If you want to use the default stderr and longjump method,
     * you can supply NULL for the last three parameters.  We also check that
     * the library version is compatible with the one used at compile time,
     * in case we are using dynamically linked libraries.  REQUIRED.
     */
    png_ptr = png_create_write_struct(PNG_LIBPNG_VER_STRING, NULL, NULL, NULL);
    if (png_ptr == NULL) {
        fclose(p_png_file);
        return ret;
    }

    /* Allocate/initialize the image information data.  REQUIRED */
    info_ptr = png_create_info_struct(png_ptr);
    if (info_ptr == NULL) {
        fclose(p_png_file);
        png_destroy_write_struct(&png_ptr, NULL);
        return ret;
    }

    /* Set error handling.  REQUIRED if you aren't supplying your own
     * error handling functions in the png_create_write_struct() call.
     */
    if (setjmp(png_jmpbuf(png_ptr))) {
        /* If we get here, we had a problem writing the file */
        fclose(p_png_file);
        png_destroy_write_struct(&png_ptr, &info_ptr);
        return ret;
    }

    /* Set up the output control if you are using standard C streams */
    png_init_io(png_ptr, p_png_file);

    /* Set the image information here.  Width and height are up to 2^31,
     * bit_depth is one of 1, 2, 4, 8, or 16, but valid values also depend on
     * the color_type selected. color_type is one of PNG_COLOR_TYPE_GRAY,
     * PNG_COLOR_TYPE_GRAY_ALPHA, PNG_COLOR_TYPE_PALETTE, PNG_COLOR_TYPE_RGB,
     * or PNG_COLOR_TYPE_RGB_ALPHA.  interlace is either PNG_INTERLACE_NONE or
     * PNG_INTERLACE_ADAM7, and the compression_type and filter_type MUST
     * currently be PNG_COMPRESSION_TYPE_BASE and PNG_FILTER_TYPE_BASE. REQUIRED
     */
    if (is_colour) {
        png_set_IHDR(png_ptr, info_ptr, width, height, bytes_per_sample * 8,
                PNG_COLOR_TYPE_RGB,
                PNG_INTERLACE_NONE, PNG_COMPRESSION_TYPE_BASE,
                PNG_FILTER_TYPE_DEFAULT);
        uint32_t profile_len;
        const unsigned char *profile = get_sRGB_profile_data(&profile_len);

        if (profile_len > 0) {
            png_set_iCCP(png_ptr, info_ptr, *name ? name : "icc", 0, (png_const_bytep) profile, profile_len);
        }
    } else {
        png_set_IHDR(png_ptr, info_ptr, width, height, bytes_per_sample * 8,
                PNG_COLOR_TYPE_GRAY,
                PNG_INTERLACE_NONE, PNG_COMPRESSION_TYPE_BASE,
                PNG_FILTER_TYPE_DEFAULT);
    }

    /* Write the file header information.  REQUIRED */
    png_write_info(png_ptr, info_ptr);

    png_bytep *row_pointers = malloc((size_t) height * sizeof(png_bytep));

    int samples_per_pixel;
    if (is_colour) {
        samples_per_pixel = 3;
    } else {
        samples_per_pixel = 1;
    }

    if (bytes_per_sample == 2) {
        /* swap bytes of 16 bit files to most significant bit first */
        png_set_swap(png_ptr);
        WORD *data = convert_data(fit);
        for (unsigned i = 0, j = height - 1; i < height; i++)
            row_pointers[j--] = (png_bytep) ((uint16_t*) data + (size_t) samples_per_pixel * i * width);
    } else {
        uint8_t *data = convert_data8(fit);
        for (unsigned i = 0, j = height - 1; i < height; i++)
            row_pointers[j--] = (uint8_t*) data + (size_t) samples_per_pixel * i * width;
    }

    png_write_image(png_ptr, row_pointers);

    /* Clean up after the write, and free any memory allocated */
    png_destroy_write_struct(&png_ptr, &info_ptr);


    /* Close the file */
    fclose(p_png_file);
    free(row_pointers);
    free(filename);
    return 0;
}

有人可以指出我的错误吗？无法读取图像的软件只会显示一个错误对话框。就这样。所以我很难知道错误在哪里。

Answer 1

我认为png_write_image()只写入了图像行数据，因此缺少各种headers或meta-data的元素。我通常使用 png_write_png() 来写整个文件。

我附上了一些对我来说绝对有用的代码，因为 Gimp 等可以读取输出。我不声称它是 production-quality ;)

int bitmap_write_png (const bitmap_t *bitmap, const char *path)
  {
  int ret = -1;

  size_t x, y;
  int pixel_size = 3;
  int depth = 8;

  FILE *fp = fopen (path, "wb");
  if (fp)
    {
    ret = 0;
    png_structp png_ptr = png_create_write_struct (PNG_LIBPNG_VER_STRING,
      NULL, NULL, NULL); 
    png_infop info_ptr = info_ptr = png_create_info_struct (png_ptr);

    png_set_IHDR (png_ptr,
                  info_ptr,
                  bitmap->width,
                  bitmap->height,
                  depth,
                  PNG_COLOR_TYPE_RGB,
                  PNG_INTERLACE_NONE,
                  PNG_COMPRESSION_TYPE_DEFAULT,
                  PNG_FILTER_TYPE_DEFAULT);

    png_byte ** row_pointers = png_malloc (png_ptr,
        bitmap->height * sizeof (png_byte *));
    for (y = 0; y < bitmap->height; y++)
      {
      png_byte *row =  
            png_malloc (png_ptr, sizeof (uint8_t) * bitmap->width * pixel_size);
      row_pointers[y] = row;
      for (x = 0; x < bitmap->width; x++)
        {
        pixel_t *pixel = pixel_at_const (bitmap, x, y);
        *row++ = pixel->red * 255;
        *row++ = pixel->green * 255;
        *row++ = pixel->blue * 255;
        }
      }

    png_init_io (png_ptr, fp);
    png_set_rows (png_ptr, info_ptr, row_pointers);
    png_write_png (png_ptr, info_ptr, PNG_TRANSFORM_IDENTITY, NULL);

    for (y = 0; y < bitmap->height; y++)
      {
      png_free (png_ptr, row_pointers[y]);
      }
    png_free (png_ptr, row_pointers);
    close (fp);
    }
  else
    {
    ret = errno;
    }
  return ret;
  }

Answer 2

根据@MarkSetchell 的建议，缺少一行：

/* Clean up after the write, and free any memory allocated */
    png_write_end(png_ptr, info_ptr); // this line was missing
    png_destroy_write_struct(&png_ptr, &info_ptr);

就这些了。

谢谢。

PNG 文件被截断并且不能被每个应用程序显示

PNG file are truncated and cannot be shown by every application

c

image

libpng