Punch Card Reader - The Software

Note: The FAQ now includes a script that can generate punch card images from text. I guess this new script could be used as a basis for a card maker - or to create punch card t-shirt logos?

When I originally started the Punch Card Reader project my first step was to obtain a few sample images by holding a camera in one hand, and a punch-card up to the window in the other. I then located detailed card specifications at http://www.quadibloc.com/comp/cardint.htm, a site that documents all the essential dimensions along with quite a bit of background history. Using these dimensions I was able to experiment with the sample images by using the Python Image Library (PIL - python 2.7). PIL makes it very easy to walk the x/y grid of an image inspecting each pixel's RGB values.
I tried to come up with a heuristic to recognise the card-edges and the punched-holes. Initially I accumulated brightness values across the entire surface and averaged them into discrete rows and columns.  This worked reasonably well but was quite slow. I soon realised that recognising the tall horizontal rows required far less precision than the smaller and more numerous vertical columns. I was able to shortcut the vertical scan by just examining a one pixel wide line across the estimated middle of each row. You can get some feel for the tolerances required from the following debug dump:

The faint red marks show where the scanning algorithm has decided it has found an edge or a hole. The faint blue rectangles plot where a holes were expected to located - you can see the the vertical drift isn't going to be much of a problem so long as the image is reasonably square and flat.  Notice that the red marks at the start of each horizontal row exhibit some drift from the true vertical and the script has to compensate for this to maintain an accurate allocation of holes to the correct columns.  On the other hand I found it adequate to calibrate the vertical height from one reading only - this is why the guide rails have holes cut in the middle - the middle reading is clearly marked on this image.

The final script accepts some parameters to help it adjust to the characteristics of the scanning hardware and to enable some debugging feedback, here is a summary of the script's parameters output by its help option:

 % python punchcard.py --help
Usage: punchcard.py [options] image [image...]
    decode punch card image into ASCII.

Options:
  -h, --help            show this help message and exit
  -b BRIGHT, --bright-threshold=BRIGHT
                        Brightness (R+G+B)/3, e.g. 127.
  -s SIDE_MARGIN_RATIO, --side-margin-ratio=SIDE_MARGIN_RATIO
                        Manually set side margin ratio (sideMargin/cardWidth).
  -d, --dump            Output an ASCII-art version of the card.
  -i, --display-image   Display an anotated version of the image.
  -r, --dump-raw        Output ASCII-art with raw row/column accumulator
                        values.
  -x XSTART, --x-start=XSTART
                        Start looking for a card edge at y position (pixels)
  -X XSTOP, --x-stop=XSTOP
                        Stop looking for a card edge at y position
  -y YSTART, --y-start=YSTART
                        Start looking for a card edge at y position
  -Y YSTOP, --y-stop=YSTOP
                        Stop looking for a card edge at y position
  -a XADJUST, --adjust-x=XADJUST
                        Adjust middle edge detect location (pixels) 

To assist with adjusting the scan for the best results the script can optionally display marked up images (seen above). Plus the script can produce an ASCII art dump, for example:

           SLAX 1            MOVE ALL CHARS ONE LEFT                            
 Card Dump of Image file: mix1/img_1961.jpg Format Dump threshold= 190
 123456789-123456789-123456789-123456789-123456789-123456789-123456789-123456789-
 ________________________________________________________________________________ 
/           SLAX 1            MOVE ALL CHARS ONE LEFT                            |
|.............O..................O.O...OOO.....O..OO.............................|
|............O................OO....OO....O..OO..O...............................|
|...........O..O................O..........O........O............................|
|.............O..O.................O.....O.......................................|
|...........O..............................O.....................................|
|............O......................OO.O.........O..O............................|
|.............................O..................................................|
|...............................OO............OO..O..............................|
|..............................O.............O.....O.............................|
|..............O.................................................................|
|.......................................O........................................|
|.........................................O......................................|
`--------------------------------------------------------------------------------'
 123456789-123456789-123456789-123456789-123456789-123456789-123456789-123456789-

That concludes this brief overview of the recognition script. The next post will describe the hardware in more detail. Full script code follows below.

The code (punchcard.py):

#!/usr/bin/env python
#
# punchcard.py 
#
# Copyright (C) 2011: Michael Hamilton
# The code is GPL 3.0(GNU General Public License) ( http://www.gnu.org/copyleft/gpl.html )
#
import Image
import sys
from optparse import OptionParser

CARD_COLUMNS = 80
CARD_ROWS = 12

# found measurements at http://www.quadibloc.com/comp/cardint.htm
CARD_WIDTH = 7.0 + 3.0/8.0 # Inches
CARD_HEIGHT = 3.25 # Inches
CARD_COL_WIDTH = 0.087 # Inches
CARD_HOLE_WIDTH = 0.055 # Inches IBM, 0.056 Control Data
CARD_ROW_HEIGHT = 0.25 # Inches
CARD_HOLE_HEIGHT = 0.125 # Inches
CARD_TOPBOT_MARGIN = 3.0/16.0 # Inches at top and bottom
CARD_SIDE_MARGIN = 0.2235 # Inches on each side


CARD_SIDE_MARGIN_RATIO = CARD_SIDE_MARGIN/CARD_WIDTH # as proportion of card width (margin/width)
CARD_TOP_MARGIN_RATIO = CARD_TOPBOT_MARGIN/CARD_HEIGHT # as proportion of card height (margin/height)
CARD_ROW_HEIGHT_RATIO = CARD_ROW_HEIGHT/CARD_HEIGHT # as proportion of card height - works
CARD_COL_WIDTH_RATIO = CARD_COL_WIDTH/CARD_WIDTH # as proportion of card height - works
CARD_HOLE_HEIGHT_RATIO = CARD_HOLE_HEIGHT/CARD_HEIGHT # as proportion of card height - works
CARD_HOLE_WIDTH_RATIO = CARD_HOLE_WIDTH/CARD_WIDTH # as a proportion of card width

BRIGHTNESS_THRESHOLD = 200  # pixel brightness value (i.e. (R+G+B)/3)

IBM_MODEL_029_KEYPUNCH = """
    /&-0123456789ABCDEFGHIJKLMNOPQR/STUVWXYZ:#@'="`.<(+|!$*);^~,%_>? |
12 / O           OOOOOOOOO                        OOOOOO             |
11|   O                   OOOOOOOOO                     OOOOOO       |
 0|    O                           OOOOOOOOO                  OOOOOO |
 1|     O        O        O        O                                 |
 2|      O        O        O        O       O     O     O     O      |
 3|       O        O        O        O       O     O     O     O     |
 4|        O        O        O        O       O     O     O     O    |
 5|         O        O        O        O       O     O     O     O   |
 6|          O        O        O        O       O     O     O     O  |
 7|           O        O        O        O       O     O     O     O |
 8|            O        O        O        O OOOOOOOOOOOOOOOOOOOOOOOO |
 9|             O        O        O        O                         | 
  |__________________________________________________________________|"""

translate = None
if translate == None:
    translate = {}
    # Turn the ASCII art sideways and build a hash look up for 
    # column values, for example:
    #   (O, , ,O, , , , , , , , ):A
    #   (O, , , ,O, , , , , , , ):B
    #   (O, , , , ,O, , , , , , ):C
    rows = IBM_MODEL_029_KEYPUNCH[1:].split('\n');
    rotated = [[ r[i] for r in rows[0:13]] for i in range(5, len(rows[0]) - 1)]
    for v in rotated:
        translate[tuple(v[1:])] = v[0]
    #print translate

# generate a range of floats
def drange(start, stop, step=1.0):
    r = start
    while (step >= 0.0 and r < stop) or (step < 0.0 and r > stop):
        yield r
        r += step

# Represents a punchcard image plus scanned data
class PunchCard(object):
    
    def __init__(self, image, bright=-1, debug=False, xstart=0, xstop=0, ystart=0, ystop=0, xadjust=0):
        pass
        self.text = ''
        self.decoded = []
        self.surface = [] 
        self.debug = debug
        self.threshold = 0
        self.ymin = ystart
        self.ymax = ystop
        self.xmin = xstart
        self.xmax = xstop
        self.xadjust = xadjust
        self.image = image
        self.pix = image.load()
        self._crop()
        self._scan(bright)
    
    # Brightness is the average of RGB values
    def _brightness(self, pixel):
        #print max(pixel)
        return ( pixel[0] + pixel[1] + pixel[2] ) / 3

    # For highlighting on the debug dump
    def _flip(self, pixel):
        return max(pixel)

    # The search is started from the "crop" edges.
    # Either use crop boundary of the image size or the valyes supplied
    # by the command line args
    def _crop(self):
        self.xsize, self.ysize = image.size
        if self.xmax == 0:
            self.xmax = self.xsize
        if self.ymax == 0:
            self.ymax = self.ysize
        self.midx = self.xmin + (self.xmax - self.xmin) / 2 + self.xadjust
        self.midy = self.ymin + (self.ymax - self.ymin) / 2

    # heuristic for finding a reasonable cutoff brightness
    def _find_threshold_brightness(self):
        left = self._brightness(self.pix[self.xmin, self.midy])
        right = self._brightness(self.pix[self.xmax - 1, self.midy])
        return min(left, right, BRIGHTNESS_THRESHOLD) - 10
        vals = []
        last = 0
        for x in xrange(self.xmin,self.xmax):
            val = self._brightness(self.pix[x, self.midy])
            if val > last:
                left = val
            else:
                break
            last = val
        for x in xrange(self.xmax,self.xmin, -1):
            val = self._brightness(self.pix[x, self.midy])
            if val > last:
                right = val
            else:
                break
            right = val
        print left, right
        return min(left, right,200)
        
        for x in xrange(self.xmin,self.xmax):
            val = self._brightness(self.pix[x, self.midy])
            vals.append(val)
        vals.sort()
        last_val = vals[0]
        biggest_diff = 0
        threshold = 0
        for val in vals:
            diff = val - last_val
            #print val, diff
            if val > 127 and val < 200 and diff >= 5:
                biggest_diff = diff
                threshold = val
            last_val = val
        if self.debug:
            print "Threshold diff=", biggest_diff, "brightness=", val
        return threshold - 10
    
    # Find the left and right edges of the data area at probe_y and from that
    # figure out the column and hole vertical dimensions at probe_y.
    def _find_data_horiz_dimensions(self, probe_y):
        left_border, right_border = self.xmin, self.xmax - 1
        for x in xrange(self.xmin, self.midx):            
            if self._brightness(self.pix[x,  probe_y]) < self.threshold:
                left_border = x
                break
        for x in xrange(self.xmax-1,  self.midx,  -1):
            if self._brightness(self.pix[x,  probe_y]) < self.threshold:
                right_border = x
                break
        width = right_border - left_border
        card_side_margin_width = int(width * CARD_SIDE_MARGIN_RATIO)
        data_left_x = left_border + card_side_margin_width
        #data_right_x = right_border - card_side_margin_width
        data_right_x = data_left_x + int((CARD_COLUMNS * width) * CARD_COL_WIDTH/CARD_WIDTH)
        col_width = width * CARD_COL_WIDTH_RATIO
        hole_width = width * CARD_HOLE_WIDTH_RATIO
        #print col_width
        if self.debug:
            # mark left and right edges on the copy
            for y in xrange(probe_y - self.ysize/100, probe_y + self.ysize/100):
                self.debug_pix[left_border if left_border > 0 else 0,y] = 255
                self.debug_pix[right_border if right_border < self.xmax else self.xmax - 1,y] = 255
            for x in xrange(1, (self.xmax - self.xmin) / 200):
                self.debug_pix[left_border + x, probe_y] = 255
                self.debug_pix[right_border - x, probe_y] = 255
                
        return data_left_x, data_right_x,  col_width, hole_width
 
    # find the top and bottom of the data area and from that the 
    # column and hole horizontal dimensions 
    def _find_data_vert_dimensions(self):
        top_border, bottom_border = self.ymin, self.ymax
        for y in xrange(self.ymin, self.midy):
            #print pix[midx,  y][0] 
            if self._brightness(self.pix[self.midx,  y]) < self.threshold:
                top_border = y
                break
        for y in xrange(self.ymax - 1,  self.midy, -1):
            if self._brightness(self.pix[self.midx,  y]) < self.threshold:
                bottom_border = y
                break
        card_height = bottom_border - top_border
        card_top_margin = int(card_height * CARD_TOP_MARGIN_RATIO)
        data_begins = top_border + card_top_margin
        hole_height = int(card_height * CARD_HOLE_HEIGHT_RATIO)
        data_top_y = data_begins + hole_height / 2
        col_height = int(card_height * CARD_ROW_HEIGHT_RATIO)
        if self.debug:
            # mark up the copy with the edges
            for x in xrange(self.xmin, self.xmax-1):
                self.debug_pix[x,top_border] = 255
                self.debug_pix[x,bottom_border] = 255
        if self.debug:
            # mark search parameters 
            for x in xrange(self.midx - self.xsize/20, self.midx + self.xsize/20):
               self.debug_pix[x,self.ymin] = 255
               self.debug_pix[x,self.ymax - 1] = 255
            for y in xrange(0, self.ymin):
               self.debug_pix[self.midx,y] = 255
            for y in xrange(self.ymax - 1, self.ysize-1):
               self.debug_pix[self.midx,y] = 255
        return data_top_y, data_top_y + col_height * 11, col_height, hole_height

    def _scan(self, bright=-1):
        if self.debug:
            # if debugging make a copy we can draw on
            self.debug_image = self.image.copy()
            self.debug_pix = self.debug_image.load()
            
        self.threshold = bright if bright > 0 else self._find_threshold_brightness()    
        #x_min, x_max,  col_width = self._find_data_horiz_dimensions(image, pix, self.threshold, self.ystart, self.ystop)
        y_data_pos, y_data_end, col_height, hole_height = self._find_data_vert_dimensions()
        data = {}
        
        # Chads are narrow so find then heuristically by accumulating pixel brightness
        # along the row.  Should be forgiving if the image is slightly wonky.
        y = y_data_pos #- col_height/8
        for row_num in xrange(CARD_ROWS):
            probe_y = y + col_height if row_num == 0 else ( y - col_height if row_num == CARD_ROWS -1 else y )  # Line 0 has a corner missing
            x_data_left, x_data_right,  col_width, hole_width = self._find_data_horiz_dimensions(probe_y)
            left_edge = -1 # of a punch-hole
            for x in xrange(x_data_left,  x_data_right):
                # Chads are tall so we can be sure if we probe around the middle of their height
                val = self._brightness(self.pix[x, y])
                if val >= self.threshold:
                    if left_edge == -1:
                        left_edge = x
                    if self.debug:
                        self.debug_pix[x,y] = self._flip(self.pix[x,y])
                else:
                    if left_edge > -1:
                        hole_length = x - left_edge
                        if hole_length >= hole_width * 0.75:
                            col_num = int((left_edge + hole_length / 2.0 - x_data_left) / col_width + 0.25)  
                            data[(col_num, row_num)] = hole_length
                        left_edge = -1
            if (self.debug):
                # Plot where holes might be on this row
                expected_top_edge = y - hole_height / 2
                expected_bottom_edge = y + hole_height / 2
                blue = 255 * 256 * 256
                for expected_left_edge in drange(x_data_left, x_data_right - 1, col_width):
                    for y_plot in drange(expected_top_edge, expected_bottom_edge, 2):
                        self.debug_pix[expected_left_edge,y_plot] = blue
                        #self.debug_pix[x + hole_width/2,yline] = 255 * 256 * 256
                        self.debug_pix[expected_left_edge + hole_width,y_plot] = blue
                    for x_plot in drange(expected_left_edge, expected_left_edge + hole_width):
                        self.debug_pix[x_plot, expected_top_edge] = blue
                        self.debug_pix[x_plot, expected_bottom_edge] = blue
            y += col_height

        if self.debug:
            self.debug_image.show()
            # prevent run-a-way debug shows causing my desktop to run out of memory
            raw_input("Press Enter to continue...")
        self.decoded = []
        # Could fold this loop into the previous one - but would it be faster?
        for col in xrange(0, CARD_COLUMNS):
            col_pattern = []
            col_surface = []
            for row in xrange(CARD_ROWS):
                key = (col, row)
                # avergage for 1/3 of a column is greater than the threshold
                col_pattern.append('O' if key in data else ' ')
                col_surface.append(data[key] if key in data else 0)
            tval = tuple(col_pattern)
            global translate
            self.text += translate[tval] if tval in translate else '@'
            self.decoded.append(tval)
            self.surface.append(col_surface)
           

        return self

    # ASCII art image of card
    def dump(self, id, raw_data=False):
        print ' Card Dump of Image file:', id, 'Format', 'Raw' if raw_data else 'Dump', 'threshold=', self.threshold
        print ' ' + '123456789-' * (CARD_COLUMNS/10)
        print ' ' + '_' * CARD_COLUMNS + ' '
        print '/' + self.text +  '_' * (CARD_COLUMNS - len(self.text)) + '|'
        for rnum in xrange(len(self.decoded[0])):
            sys.stdout.write('|')
            if raw_data:
                for val in self.surface:
                    sys.stdout.write(("(%d)" % val[rnum]) if val[rnum] != 0 else '.' )
            else:
                for col in self.decoded:
                    sys.stdout.write(col[rnum] if col[rnum] == 'O' else '.')
            print '|'
        print '`' + '-' * CARD_COLUMNS + "'"
        print ' ' + '123456789-' * (CARD_COLUMNS/10)
        print ''
         
            
if __name__ == '__main__':
    
    usage = """usage: %prog [options] image [image...]
    decode punch card image into ASCII."""
    parser = OptionParser(usage)
    parser.add_option('-b', '--bright-threshold', type='int', dest='bright', default=-1, help='Brightness (R+G+B)/3, e.g. 127.')
    parser.add_option('-s', '--side-margin-ratio', type='float', dest='side_margin_ratio', default=CARD_SIDE_MARGIN_RATIO, help='Manually set side margin ratio (sideMargin/cardWidth).')
    parser.add_option('-d', '--dump', action='store_true', dest='dump', help='Output an ASCII-art version of the card.')
    parser.add_option('-i', '--display-image', action='store_true', dest='display', help='Display an anotated version of the image.')
    parser.add_option('-r', '--dump-raw', action='store_true', dest='dumpraw', help='Output ASCII-art with raw row/column accumulator values.')
    parser.add_option('-x', '--x-start', type='int', dest='xstart', default=0, help='Start looking for a card edge at y position (pixels)')
    parser.add_option('-X', '--x-stop', type='int', dest='xstop', default=0, help='Stop looking for a card edge at y position')
    parser.add_option('-y', '--y-start', type='int', dest='ystart', default=0, help='Start looking for a card edge at y position')
    parser.add_option('-Y', '--y-stop', type='int', dest='ystop', default=0, help='Stop looking for a card edge at y position')
    parser.add_option('-a', '--adjust-x', type='int', dest='xadjust', default=0, help='Adjust middle edge detect location (pixels)')
    (options, args) = parser.parse_args()
    
    for arg in args:
        image = Image.open(arg)
        card = PunchCard(image,  bright=options.bright, debug=options.display, xstart=options.xstart, xstop=options.xstop, ystart=options.ystart, ystop=options.ystop, xadjust=options.xadjust)
        print card.text
        if (options.dump):
            card.dump(arg)
        if (options.dumpraw):
            card.dump(arg, raw_data=True)