check_transparent

#!/usr/bin/env python2.6

import cStringIO, datetime, os, glob, re, subprocess, sys
from optparse import OptionParser
from alppng import *

def my_exec( cmd, no_assert = False ):
    pipe = subprocess.Popen( ( cmd if isinstance( cmd, str ) else ' '.join( cmd ) ),
                                     shell=True,
                                     stdin=subprocess.PIPE,
                                     stdout=subprocess.PIPE,
                                     stderr=subprocess.PIPE )
    ret = ( pipe.stdout.read(), pipe.stderr.read(), pipe.wait(), )

    if not no_assert:
        assert ret[2] == 0

    return ret


def main( argv ):

    parser = OptionParser(usage="""usage: %prog file

    Examples:

    check_transparent <filename>

    Checks if a png image file contains some transparency.
    Returns 1 if yes, 0 otherwise.
    """)

    (options, args) = parser.parse_args()
    argc = len( args )

    if argc == 0:
        parser.print_help()
        sys.exit(1)

    filename = args[ 0 ]

    png     = alppng.Png( filename )
    ihdr    = alppng.Png_get_ihdr_chunk( png )
    npixels = ihdr[ alppng.WIDTH ] * ihdr[ alppng.HEIGHT ]

    try:

        b = alppng.Png_has_plte_index_and_is_transparent( png )

        if b == True:   # Transparent
            sys.exit(1)

        if b == False:  # Opaque
            sys.exit(0)

    except AssertionError, e:    # e.g. when filter_type != 0   (rare)
        pass

    # At this point, ALPPNG says : "may be opaque, may be transparent, I do not know".

    IDENTIFY = '/usr/bin/identify'
    if not os.path.exists( IDENTIFY ):
        r = my_exec( '/usr/bin/which identify' )
        IDENTIFY = ( r[0] if ( r[2] == 0 ) else None )

    rx_alpha = re.compile( '^\s*[Aa]lpha\s*:\s*(\d+)-bit\W', re.MULTILINE | re.IGNORECASE )

    # Try the fallback: Image Magick
    if not IDENTIFY:
        sys.exit(0)

    out,err,code = my_exec( IDENTIFY + ' -verbose ' + filename, True )

    if code != 0:
        sys.exit(0)

    mo_alpha = rx_alpha.search( out )

    if mo_alpha == None:
        # Image Magick says: is opaque.
        sys.exit(0)

    # Now we are sure that the image has an alpha channel.

    # Let us check the histogram first, maybe we can say right away that the image is fully transparent.

    rx_full_transp = re.compile( r'Histogram\s*:[\s\r\n]*{npixels}\s*:\s+\(\s*\d+\s*,\s*\d+\s*,\s*\d+\s*,\s*0\s*\)\s+#[a-zA-Z0-9]{{6,6}}00\s+'.format( **vars() ) )

    if None == rx_full_transp.search( out ):

        # Now let us check the rest of the information, to detect non-transparent pixels.

        rx_alpha_opaque_1bit = re.compile( '^[\s\r\n]*Alpha[\s\r\n]*:[\s\r\n]*min[\s\r\n]*:[\s\r\n]*0[\s\r\n]*\(0\)[\s\r\n]*max[\s\r\n]*:[\s\r\n]*0[\s\r\n]*\(0\)[\s\r\n]+', re.MULTILINE | re.IGNORECASE )
        rx_alpha_opaque = re.compile( '^[\s\r\n]*Alpha[\s\r\n]*:[\s\r\n]*min[\s\r\n]*:[\s\r\n]*255[\s\r\n]*\(1\)[\s\r\n]*max[\s\r\n]*:[\s\r\n]*255[\s\r\n]*\(1\)[\s\r\n]+', re.MULTILINE | re.IGNORECASE )
        rx_alpha_opaque_2 = re.compile( '^[\s\r\n]*Opacity[\s\r\n]*:[\s\r\n]*min[\s\r\n]*:[\s\r\n]*0[\s\r\n]*\(0\)[\s\r\n]*max[\s\r\n]*:[\s\r\n]*0[\s\r\n]*\(0\)[\s\r\n]+', re.MULTILINE | re.IGNORECASE )

        im_says_opaque = False
        if mo_alpha.group(1) == '1':   # 1-bit case
            im_says_opaque = rx_alpha_opaque_1bit.search( out ) != None

        if not im_says_opaque:
           im_says_opaque = (rx_alpha_opaque.search( out ) != None) or (rx_alpha_opaque_2.search( out ) != None)

        if im_says_opaque:
            # Image Magick says: has opaque content.
            # (has alpha channel with: min = max = 255
            # or opacity with: min = max = 0).
            sys.exit(0)

    sys.exit(1)


if __name__ == '__main__':
    main( sys.argv )