readpng2.c

png图像文件格式开发包

    /* make sure we save our pointers for use in readpng2_decode_data() */

    mainprog_ptr->png_ptr = png_ptr;
    mainprog_ptr->info_ptr = info_ptr;


    /* and that's all there is to initialization */

    return 0;
}




/* returns 0 for success, 2 for libpng (longjmp) problem */

int readpng2_decode_data(mainprog_info *mainprog_ptr, uch *rawbuf, ulg length)
{
    png_structp png_ptr = (png_structp)mainprog_ptr->png_ptr;
    png_infop info_ptr = (png_infop)mainprog_ptr->info_ptr;


    /* setjmp() must be called in every function that calls a PNG-reading
     * libpng function */

    if (setjmp(mainprog_ptr->jmpbuf)) {
        png_destroy_read_struct(&png_ptr, &info_ptr, NULL);
        mainprog_ptr->png_ptr = NULL;
        mainprog_ptr->info_ptr = NULL;
        return 2;
    }


    /* hand off the next chunk of input data to libpng for decoding */

    png_process_data(png_ptr, info_ptr, rawbuf, length);

    return 0;
}




static void readpng2_info_callback(png_structp png_ptr, png_infop info_ptr)
{
    mainprog_info  *mainprog_ptr;
    int  color_type, bit_depth;
    double  gamma;


    /* setjmp() doesn't make sense here, because we'd either have to exit(),
     * longjmp() ourselves, or return control to libpng, which doesn't want
     * to see us again.  By not doing anything here, libpng will instead jump
     * to readpng2_decode_data(), which can return an error value to the main
     * program. */


    /* retrieve the pointer to our special-purpose struct, using the png_ptr
     * that libpng passed back to us (i.e., not a global this time--there's
     * no real difference for a single image, but for a multithreaded browser
     * decoding several PNG images at the same time, one needs to avoid mixing
     * up different images' structs) */

    mainprog_ptr = png_get_progressive_ptr(png_ptr);

    if (mainprog_ptr == NULL) {         /* we be hosed */
        fprintf(stderr,
          "readpng2 error:  main struct not recoverable in info_callback.\n");
        fflush(stderr);
        return;
        /*
         * Alternatively, we could call our error-handler just like libpng
         * does, which would effectively terminate the program.  Since this
         * can only happen if png_ptr gets redirected somewhere odd or the
         * main PNG struct gets wiped, we're probably toast anyway.  (If
         * png_ptr itself is NULL, we would not have been called.)
         */
    }


    /* this is just like in the non-progressive case */

    png_get_IHDR(png_ptr, info_ptr, &mainprog_ptr->width,
      &mainprog_ptr->height, &bit_depth, &color_type, NULL, NULL, NULL);


    /* since we know we've read all of the PNG file's "header" (i.e., up
     * to IDAT), we can check for a background color here */

    if (mainprog_ptr->need_bgcolor &&
        png_get_valid(png_ptr, info_ptr, PNG_INFO_bKGD))
    {
        png_color_16p pBackground;

        /* it is not obvious from the libpng documentation, but this function
         * takes a pointer to a pointer, and it always returns valid red,
         * green and blue values, regardless of color_type: */
        png_get_bKGD(png_ptr, info_ptr, &pBackground);

        /* however, it always returns the raw bKGD data, regardless of any
         * bit-depth transformations, so check depth and adjust if necessary */
        if (bit_depth == 16) {
            mainprog_ptr->bg_red   = pBackground->red   >> 8;
            mainprog_ptr->bg_green = pBackground->green >> 8;
            mainprog_ptr->bg_blue  = pBackground->blue  >> 8;
        } else if (color_type == PNG_COLOR_TYPE_GRAY && bit_depth < 8) {
            if (bit_depth == 1)
                mainprog_ptr->bg_red = mainprog_ptr->bg_green =
                  mainprog_ptr->bg_blue = pBackground->gray? 255 : 0;
            else if (bit_depth == 2)
                mainprog_ptr->bg_red = mainprog_ptr->bg_green =
                  mainprog_ptr->bg_blue = (255/3) * pBackground->gray;
            else /* bit_depth == 4 */
                mainprog_ptr->bg_red = mainprog_ptr->bg_green =
                  mainprog_ptr->bg_blue = (255/15) * pBackground->gray;
        } else {
            mainprog_ptr->bg_red   = (uch)pBackground->red;
            mainprog_ptr->bg_green = (uch)pBackground->green;
            mainprog_ptr->bg_blue  = (uch)pBackground->blue;
        }
    }


    /* as before, let libpng expand palette images to RGB, low-bit-depth
     * grayscale images to 8 bits, transparency chunks to full alpha channel;
     * strip 16-bit-per-sample images to 8 bits per sample; and convert
     * grayscale to RGB[A] */

    if (color_type == PNG_COLOR_TYPE_PALETTE)
        png_set_expand(png_ptr);
    if (color_type == PNG_COLOR_TYPE_GRAY && bit_depth < 8)
        png_set_expand(png_ptr);
    if (png_get_valid(png_ptr, info_ptr, PNG_INFO_tRNS))
        png_set_expand(png_ptr);
    if (bit_depth == 16)
        png_set_strip_16(png_ptr);
    if (color_type == PNG_COLOR_TYPE_GRAY ||
        color_type == PNG_COLOR_TYPE_GRAY_ALPHA)
        png_set_gray_to_rgb(png_ptr);


    /* Unlike the basic viewer, which was designed to operate on local files,
     * this program is intended to simulate a web browser--even though we
     * actually read from a local file, too.  But because we are pretending
     * that most of the images originate on the Internet, we follow the recom-
     * mendation of the sRGB proposal and treat unlabelled images (no gAMA
     * chunk) as existing in the sRGB color space.  That is, we assume that
     * such images have a file gamma of 0.45455, which corresponds to a PC-like
     * display system.  This change in assumptions will have no effect on a
     * PC-like system, but on a Mac, SGI, NeXT or other system with a non-
     * identity lookup table, it will darken unlabelled images, which effec-
     * tively favors images from PC-like systems over those originating on
     * the local platform.  Note that mainprog_ptr->display_exponent is the
     * "gamma" value for the entire display system, i.e., the product of
     * LUT_exponent and CRT_exponent. */

    if (png_get_gAMA(png_ptr, info_ptr, &gamma))
        png_set_gamma(png_ptr, mainprog_ptr->display_exponent, gamma);
    else
        png_set_gamma(png_ptr, mainprog_ptr->display_exponent, 0.45455);


    /* we'll let libpng expand interlaced images, too */

    mainprog_ptr->passes = png_set_interlace_handling(png_ptr);


    /* all transformations have been registered; now update info_ptr data and
     * then get rowbytes and channels */

    png_read_update_info(png_ptr, info_ptr);

    mainprog_ptr->rowbytes = (int)png_get_rowbytes(png_ptr, info_ptr);
    mainprog_ptr->channels = png_get_channels(png_ptr, info_ptr);


    /* Call the main program to allocate memory for the image buffer and
     * initialize windows and whatnot.  (The old-style function-pointer
     * invocation is used for compatibility with a few supposedly ANSI
     * compilers that nevertheless barf on "fn_ptr()"-style syntax.) */

    (*mainprog_ptr->mainprog_init)();


    /* and that takes care of initialization */

    return;
}





static void readpng2_row_callback(png_structp png_ptr, png_bytep new_row,
                                  png_uint_32 row_num, int pass)
{
    mainprog_info  *mainprog_ptr;


    /* first check whether the row differs from the previous pass; if not,
     * nothing to combine or display */

    if (!new_row)
        return;


    /* retrieve the pointer to our special-purpose struct so we can access
     * the old rows and image-display callback function */

    mainprog_ptr = png_get_progressive_ptr(png_ptr);


    /* save the pass number for optional use by the front end */

    mainprog_ptr->pass = pass;


    /* have libpng either combine the new row data with the existing row data
     * from previous passes (if interlaced) or else just copy the new row
     * into the main program's image buffer */

    png_progressive_combine_row(png_ptr, mainprog_ptr->row_pointers[row_num],
      new_row);


    /* finally, call the display routine in the main program with the number
     * of the row we just updated */

    (*mainprog_ptr->mainprog_display_row)(row_num);


    /* and we're ready for more */

    return;
}





static void readpng2_end_callback(png_structp png_ptr, png_infop info_ptr)
{
    mainprog_info  *mainprog_ptr;


    /* retrieve the pointer to our special-purpose struct */

    mainprog_ptr = png_get_progressive_ptr(png_ptr);


    /* let the main program know that it should flush any buffered image
     * data to the display now and set a "done" flag or whatever, but note
     * that it SHOULD NOT DESTROY THE PNG STRUCTS YET--in other words, do
     * NOT call readpng2_cleanup() either here or in the finish_display()
     * routine; wait until control returns to the main program via
     * readpng2_decode_data() */

    (*mainprog_ptr->mainprog_finish_display)();


    /* all done */

    return;
}





void readpng2_cleanup(mainprog_info *mainprog_ptr)
{
    png_structp png_ptr = (png_structp)mainprog_ptr->png_ptr;
    png_infop info_ptr = (png_infop)mainprog_ptr->info_ptr;

    if (png_ptr && info_ptr)
        png_destroy_read_struct(&png_ptr, &info_ptr, NULL);

    mainprog_ptr->png_ptr = NULL;
    mainprog_ptr->info_ptr = NULL;
}





static void readpng2_error_handler(png_structp png_ptr, png_const_charp msg)
{
    mainprog_info  *mainprog_ptr;

    /* This function, aside from the extra step of retrieving the "error
     * pointer" (below) and the fact that it exists within the application
     * rather than within libpng, is essentially identical to libpng's
     * default error handler.  The second point is critical:  since both
     * setjmp() and longjmp() are called from the same code, they are
     * guaranteed to have compatible notions of how big a jmp_buf is,
     * regardless of whether _BSD_SOURCE or anything else has (or has not)
     * been defined. */

    fprintf(stderr, "readpng2 libpng error: %s\n", msg);
    fflush(stderr);

    mainprog_ptr = png_get_error_ptr(png_ptr);
    if (mainprog_ptr == NULL) {         /* we are completely hosed now */
        fprintf(stderr,
          "readpng2 severe error:  jmpbuf not recoverable; terminating.\n");
        fflush(stderr);
        exit(99);
    }

    longjmp(mainprog_ptr->jmpbuf, 1);
}