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that libpng will output.  Consequently you must call all the png_set_
functions before you call png_read_update_info().  This is particularly
important for png_set_interlace_handling() - if you are going to call
png_read_update_info() you must call png_set_interlace_handling() before
it unless you want to receive interlaced output.

Reading image data

After you've allocated memory, you can read the image data.
The simplest way to do this is in one function call.  If you are
allocating enough memory to hold the whole image, you can just
call png_read_image() and libpng will read in all the image data
and put it in the memory area supplied.  You will need to pass in
an array of pointers to each row.

This function automatically handles interlacing, so you don't
need to call png_set_interlace_handling() (unless you call
png_read_update_info()) or call this function multiple times, or any
of that other stuff necessary with png_read_rows().

   png_read_image(png_ptr, row_pointers);

where row_pointers is:

   png_bytep row_pointers[height];

You can point to void or char or whatever you use for pixels.

If you don't want to read in the whole image at once, you can
use png_read_rows() instead.  If there is no interlacing (check
interlace_type == PNG_INTERLACE_NONE), this is simple:

    png_read_rows(png_ptr, row_pointers, NULL,
        number_of_rows);

where row_pointers is the same as in the png_read_image() call.

If you are doing this just one row at a time, you can do this with
a single row_pointer instead of an array of row_pointers:

    png_bytep row_pointer = row;
    png_read_row(png_ptr, row_pointer, NULL);

If the file is interlaced (interlace_type != 0 in the IHDR chunk), things
get somewhat harder.  The only current (PNG Specification version 1.2)
interlacing type for PNG is (interlace_type == PNG_INTERLACE_ADAM7);
a somewhat complicated 2D interlace scheme, known as Adam7, that
breaks down an image into seven smaller images of varying size, based
on an 8x8 grid.  This number is defined (from libpng 1.5) as
PNG_INTERLACE_ADAM7_PASSES in png.h

libpng can fill out those images or it can give them to you "as is".
It is almost always better to have libpng handle the interlacing for you.
If you want the images filled out, there are two ways to do that.  The one
mentioned in the PNG specification is to expand each pixel to cover
those pixels that have not been read yet (the "rectangle" method).
This results in a blocky image for the first pass, which gradually
smooths out as more pixels are read.  The other method is the "sparkle"
method, where pixels are drawn only in their final locations, with the
rest of the image remaining whatever colors they were initialized to
before the start of the read.  The first method usually looks better,
but tends to be slower, as there are more pixels to put in the rows.

If, as is likely, you want libpng to expand the images, call this before
calling png_start_read_image() or png_read_update_info():

    if (interlace_type == PNG_INTERLACE_ADAM7)
       number_of_passes
           = png_set_interlace_handling(png_ptr);

This will return the number of passes needed.  Currently, this is seven,
but may change if another interlace type is added.  This function can be
called even if the file is not interlaced, where it will return one pass.
You then need to read the whole image 'number_of_passes' times.  Each time
will distribute the pixels from the current pass to the correct place in
the output image, so you need to supply the same rows to png_read_rows in
each pass.

If you are not going to display the image after each pass, but are
going to wait until the entire image is read in, use the sparkle
effect.  This effect is faster and the end result of either method
is exactly the same.  If you are planning on displaying the image
after each pass, the "rectangle" effect is generally considered the
better looking one.

If you only want the "sparkle" effect, just call png_read_rows() as
normal, with the third parameter NULL.  Make sure you make pass over
the image number_of_passes times, and you don't change the data in the
rows between calls.  You can change the locations of the data, just
not the data.  Each pass only writes the pixels appropriate for that
pass, and assumes the data from previous passes is still valid.

    png_read_rows(png_ptr, row_pointers, NULL,
        number_of_rows);

If you only want the first effect (the rectangles), do the same as
before except pass the row buffer in the third parameter, and leave
the second parameter NULL.

    png_read_rows(png_ptr, NULL, row_pointers,
        number_of_rows);

If you don't want libpng to handle the interlacing details, just call
png_read_rows() PNG_INTERLACE_ADAM7_PASSES times to read in all the images.
Each of the images is a valid image by itself; however, you will almost
certainly need to distribute the pixels from each sub-image to the
correct place.  This is where everything gets very tricky.

If you want to retrieve the separate images you must pass the correct
number of rows to each successive call of png_read_rows().  The calculation
gets pretty complicated for small images, where some sub-images may
not even exist because either their width or height ends up zero.
libpng provides two macros to help you in 1.5 and later versions:

   png_uint_32 width = PNG_PASS_COLS(image_width, pass_number);
   png_uint_32 height = PNG_PASS_ROWS(image_height, pass_number);

Respectively these tell you the width and height of the sub-image
corresponding to the numbered pass.  'pass' is in in the range 0 to 6 -
this can be confusing because the specification refers to the same passes
as 1 to 7!  Be careful, you must check both the width and height before