Find the first black pixel in every row with ImageMagick - imagemagick

Question

For every row in an image, I would like to find the first black (or first non-white) pixel in that row. For example, for an image like this:
I would expect output like:
0
1
0
Or something close to that that I can parse. I think there might be a way to do this with subimage-search but I don't quite know how. Any pointers?

Answer 1

You do NOT need subimage-search to achieve your goal. The problem can be reduced to text parsing.
1. Basics
Consider this: you can tell ImageMagick to convert any image to a textual representation, which holds the exact color information for each individual pixel. Example:
convert wizard: textwizard.txt
(wizard: is a builtin image available for all ImageMagick installations for testing purposes.)
Yes, it is that easy! This image "format" is requested by just adding a .txt suffix. Results:
# ImageMagick pixel enumeration: 480,640,255,srgb
0,0: (255,255,255) #FFFFFF white
1,0: (255,255,255) #FFFFFF white
2,0: (255,255,255) #FFFFFF white
[....]
47,638: (246,247,249) #F6F7F9 srgb(246,247,249)
48,638: (246,247,249) #F6F7F9 srgb(246,247,249)
47,639: (236,235,236) #ECEBEC srgb(236,235,236)
48,639: (230,228,218) #E6E4DA srgb(230,228,218)
[....]
476,639: (255,255,255) #FFFFFF white
477,639: (255,255,255) #FFFFFF white
478,639: (255,255,255) #FFFFFF white
479,639: (255,255,255) #FFFFFF white
If you look at the first line of the output, you'll notice that ImageMagick uses it to detail some special info about the image here:
# ImageMagick pixel enumeration: 480,640,255,srgb
It means:
the image is 480 pixels wide,
the image is 640 pixels high,
the image uses a range of 0-255 for color info per channel (that is equivalent to an 8-bit color depth),
the image is build in the sRGB color space
The other lines consist of 4 columns:
the first column in format (N,M) indicates the exact position of the respective pixels as (row_number,column_number). (The index for row and column numbers is zero-based -- row no. 1 is indicated as 0, no. 2 as 1.)
the other three columns, redundantly, each hold the exact same information, each in a different notation: the exact color value for the pixel given in column 1. (The last column will use a human-readable name if ImageMagick knows one for that color value...)
As a side note: you can use such a textual representation of the original image (with or without some extra modifications) to re-create a real image:
convert textwizard.txt wizard.jpg
2. Select a specific row
You should be aware that you can select a specific region of an image with the following syntax:
image.png[WIDTHxHEIGHT+X_OFFSET+Y_OFFSET]
So to select a specific row only, you can set HEIGHT as 1. To get any row completely, set X-OFFSET as 0. To get a specific row, set the Y-OFFSET accordingly.
In order to get the values (for the builtin wizard: image used above) for the row with index 47, we can do:
convert wizard:[640x1+0+47] row47.txt
cat row47.txt
# ImageMagick pixel enumeration: 480,1,255,srgb
0,0: (255,255,255) #FFFFFF white
1,0: (255,255,255) #FFFFFF white
2,0: (255,255,255) #FFFFFF white
[....]
428,0: (82,77,74) #524D4A srgb(82,77,74)
429,0: (169,167,168) #A9A7A8 srgb(169,167,168)
430,0: (232,231,228) #E8E7E4 srgb(232,231,228)
432,0: (246,247,249) #F6F7F9 srgb(246,247,249)
[....]
476,0: (255,255,255) #FFFFFF white
477,0: (255,255,255) #FFFFFF white
478,0: (255,255,255) #FFFFFF white
479,0: (255,255,255) #FFFFFF white
If you do not want the textual output in a file, but printed on the standard output channel, you can do this:
convert wizard:[480x1+0+47] txt:-
3. Stitching it all together
Based on above snippets of information, the approach that can be taken with this task is clear:
Loop through all pixel rows of the image.
Output each pixel's color value as text.
Look for the first non-white pixel and keep its location information.
4. Possible script (OS X, Linux, Unix)
Here is a main part of a Bash script that could be used:
# Define some image specific variables (width, height, ...)
image=${1}
number_of_columns=$(identify -format '%W' ${image})
width=${number_of_columns} # just an alias
number_of_rows=$(identify -format '%H' ${image})
height=${number_of_rows} # just an alias
max_of_indices=$(( ${height} -1 ))
# Loop through all rows and grep for first non-white pixel
for i in $(seq 0 ${max_of_indices}); do
echo -n "Row ${i} : " ;
convert ${image}[${width}x1+0+${i}] txt:- \
| grep -v enumeration \
| grep -v '#FFFFFF' -m 1 \
|| echo "All WHITE pixels in row!"
done
The -v white will de-select all lines which contain the string white.
The -m 1 parameter will return the maximum of 1 matches (i.e. the first match).
It will be slow, but it will work.

Answer 2

I would go with something like this using the built-in checkerboard pattern:
convert -size 100x100 pattern:checkerboard -auto-level board.png
#!/bin/bash
convert wizard: txt: | awk -F'[,: ]' '
/^#/ || /#FFFFFF/ {next}
!($2 in fb) {fb[$2]=$1}
END {r=$2;for(i=0;i<=r;i++){if(i in fb)print i,fb[i]; else print i,"-1"}}'
The -F[,: ] tells awk to split the words on the line by commas, colons or spaces - this helps me get at the row and column at the start of each line. The line with /^#/ skips the comment in the first line of ImageMagick text output and all lines that contain white or #FFFFFF.
Then, I have an array fb[] , indexed by image row, that holds the column of the first black pixel on each row. Each time I find a line with a row not in my array fb[], I save it in the array.
At the end, inside END{}, I run through fb[] printing all rows and indices of first black pixels in those rows. Note that I output -1 in place of any undefined elements (i.e. those with no non-white pixels) - thanks to #KurtPfeifle for the hint.