imagemagick detect coordinates of transparent areas - ruby-on-rails

Question

I have a PNG image with transparent areas, squares/rectangle areas that contains transparency.
I would like to know if there is some way that i can know the top,lef,width,height of theses transparent areas in the image.
Thanks for any help

Answer 1

Updated Answer
In the intervening years, I have come across a simpler solution, so I thought I would update it so anyone else seeing it can get the benefit of the latest and greatest.
Start off just the same, by extracting the alpha layer to an image of its own and invert it:
convert start.png -alpha extract -negate intermediate.png
Now perform a "Connected Component Analysis" on that:
convert start.png -alpha extract -negate \
-define connected-components:verbose=true \
-define connected-components:area-threshold=100 \
-connected-components 8 -auto-level result.png
Objects (id: bounding-box centroid area mean-color):
0: 256x256+0+0 128.7,130.4 62740 srgb(0,0,0)
3: 146x8+103+65 175.5,68.5 1168 srgb(255,255,255)
2: 9x93+29+42 33.0,88.0 837 srgb(255,255,255)
1: 113x7+4+21 60.0,24.0 791 srgb(255,255,255)
You will see there is a header line and 4 lines of output and each has a colour at the end, the first line is black, and corresponds to the entire shape, and the the last three are white, corresponding to the three transparent areas. It is basically the second field on each of the last three lines that you want. So, 146x8+103+65 means a box 146px wide by 103px tall offset 103px to the right of the top-left corner, and 65px down from the top-left corner.
If I draw those in, in red, you can see what it has identified:
convert result.png -stroke red -fill none -strokewidth 1 \
-draw "rectangle 103,65 249,73" \
-draw "rectangle 29,42 38,135" \
-draw "rectangle 4,21 117,28" result.png
Original Answer
The following may help you get to an answer but I have not developed it all the way through to completion - people often ask questions and then never log in again and there is quite a lot of effort involved...
Let's start with this input image - where the white areas are transparent:
You can extract the alpha channel from an image with ImageMagick like this:
convert input.png -alpha extract -negate alpha.png
which gives this, where white areas are transparent
Ok, one approach is to find the bounding box of the white areas, you can do this with trim and it will give you the bounding box that encloses the white areas:
convert input.png -alpha extract -format "%#" info:
245x114+4+21
So the bounding box is 245px wide and 114px high starting at offset +4+21 from top left. I can draw that on the image to show it:
So, that is a start.
Also, you can get ImageMagick to enumerate the pixels in text format, so you can run this command
convert input.png -alpha extract -negate txt: | more
# ImageMagick pixel enumeration: 256,256,255,gray
0,0: (0,0,0) #000000 gray(0)
1,0: (0,0,0) #000000 gray(0)
2,0: (0,0,0) #000000 gray(0)
which tells you that the image is 256x256 and that the first 3 pixels are all black. If you want the white ones (i.e. transparent ones) you can do this:
convert input.png -alpha extract -negate txt: | grep FFFFFF | more
4,21: (255,255,255) #FFFFFF gray(255)
5,21: (255,255,255) #FFFFFF gray(255)
6,21: (255,255,255) #FFFFFF gray(255)
7,21: (255,255,255) #FFFFFF gray(255)
This tells you that pixel 4,21 is the top left corner of your transparent area - I'm glad it matches the output from the bounding box method above :-)
So, you can easily get a list of all the pixels that are transparent. This approach could be developed, or something similar coded up in Ruby (RMagick) to find contiguous areas of black - but that is beyond the scope of this answer for the moment - as I am not a Ruby programmer :-)
Ok, I have learned some Ruby this afternoon and, no laughter please, this is my first Ruby program. It is probably pretty ugly and more like Perl or C (my preferred languages) but it works and finds the rectangular transparent areas.
#!/usr/bin/ruby
require 'RMagick'
include Magick
infile=ARGV[0]
img = ImageList.new(infile)
w=img.columns
h=img.rows
#Extract alpha channel into pixel array
px=img.export_pixels(0,0,w,h,"A")
for row in 0..h-1
for col in 0..w-1
thispx=px[w*row+col]
if thispx<32768 then
a=row
b=col
# Find extent (c) of rectangle towards right
for r in col..w-1
thispx=px[w*row+r]
if thispx<32768
c=r
else
break
end
end
# Find extent (d) of rectangle towards bottom
for s in row..h-1
thispx=px[w*s+col]
if thispx<32768
d=s
else
break
end
end
# Blank this rectangle as we have located it
for r in row..d
for s in col..c
px[w*r+s]=65535
end
end
# Tell caller about this rectangle
printf "%d,%d %d,%d\n",a,b,d,c
end
end
end
Run it like this:
bounds.rb input.png