How to perform 'SKIP'/'SEEK' action in libpng with apng patch

Question

I am trying to port libpng/apng to Android platform, using libpng with apng patch to read animated png file.

My question is that I didn't find any 'skip' method declared in png.h. What I want to do is like to directly jump to a specific frame. But I cannot get a correct result unless I read from the beginning and perform png_read_frame_head() and png_read_image() for every frame ahead.

Is there any way that can jump to a specific frame by specifying the index without read all the frame info/data ahead?

The following code is from apng sample http://littlesvr.ca/apng/tutorial/x57.html You can see it reads the apng file in a loop. And it seems that you have to call png_read_frame_head() and png_read_image() in order to make the internal information in the png_ptr_read and info_ptr_read updated. So, if there is any way to simply modify these two struct to correct information prepared for reading a specific frame, my question is solved.

for(count = 0; count < png_get_num_frames(png_ptr_read, info_ptr_read); count++)
{
    sprintf(filename, "extracted-%02d.png", count);
    newImage = fopen(filename, "wb");
    if(newImage == NULL)
        fatalError("couldn't create png for writing");

    writeSetup(newImage, &png_ptr_write, &info_ptr_write);
    if(setjmp(png_ptr_write->jmpbuf))
        fatalError("something didn't work, jump 2");

    png_read_frame_head(png_ptr_read, info_ptr_read);

    if(png_get_valid(png_ptr_read, info_ptr_read, PNG_INFO_fcTL))
    {
        png_get_next_frame_fcTL(png_ptr_read, info_ptr_read,
                                &next_frame_width, &next_frame_height,
                                &next_frame_x_offset, &next_frame_y_offset,
                                &next_frame_delay_num, &next_frame_delay_den,
                                &next_frame_dispose_op, &next_frame_blend_op);
    }
    else
    {
        /* the first frame doesn't have an fcTL so it's expected to be hidden, 
        * but we'll extract it anyway */
        next_frame_width = png_get_image_width(png_ptr_read, info_ptr_read);
        next_frame_height = png_get_image_height(png_ptr_read, info_ptr_read);
    }

    writeSetup2(png_ptr_read, info_ptr_read, png_ptr_write, info_ptr_write,
                next_frame_width, next_frame_height);

    png_write_info(png_ptr_write, info_ptr_write);

    png_read_image(png_ptr_read, rowPointers);

    png_write_image(png_ptr_write, rowPointers);

    png_write_end(png_ptr_write, NULL);

    png_destroy_write_struct(&png_ptr_write, &info_ptr_write);

    fclose(newImage);

    printf("extracted frame %d into %s\n", count, filename);
}

Answer 1

You can't. libpng was designed to treat PNG data as a stream, so it decodes chunks sequentially, one by one. Not sure why you need to skip APNG frames. Just like in video formats, one frame might be stored as "what changed after previous frame", instead of full frame, so you might need previous frame(s) too.

These code examples might be useful: https://sourceforge.net/projects/apng/files/libpng/examples/