draw grid with cells of specific size over an image

Question

How can I draw a grid, 1px black borders no fill, over an image?

Each section of the grid should be 480x360px.

E.g., given this 1440x1080px solid white input:

It should have a 3x3 grid drawn onto it (because 9 480x360px rectangles fit into it) to make an output kind of like this:

That's not as accurate as I'd like the command to be (I was just drawing rectangles by eye), but I hope it illustrates what I'm after.

You can do that in ImageMagick using the -draw "line ... " function. If on a Unix imagemagick system, you might be able to use my bash unix shell script, grid. See http://www.fmwconcepts.com/imagemagick/index.php — fmw42, Feb 06 '18 at 02:08

GeeMack · Answer 1 · 2018-02-07T16:41:08.970

Here is a command that will read an input image, create a 480x360 transparent cell with a black border, create a grid of those cells the size of the input image, and composite that grid over the input...

infile="MyPic.png"

convert "$infile" -size 480x360 -set option:distort:viewport "%[w]x%[h]" \
   \( xc:none -shave 1x1 -compose copy -bordercolor black -border 1x1 \) \
   -virtual-pixel tile -distort SRT 0 -compose over -composite output.png

That will make the lines of the grid 2 pixels thick. If the lines must be 1 pixel thick, the same sort of thing can be done with a command like this...

convert "$infile" -size 480x360 -set option:distort:viewport "%[w]x%[h]" \
   \( xc:none -chop 1x1 -background black -splice 1x1 \) \
   -virtual-pixel tile -distort SRT 0 -compose over -composite \
   -bordercolor black -shave 1x1 -compose copy -border 1x1 output.png

EDITED TO ADD: If the objective is to divide the grid into a particular number of cells rather than cells of specified dimensions, a command like this should work...

convert "$infile" \( +clone -channel A -evaluate set 0 +channel \
  -crop 3x4@ -chop 1x1 -background black -splice 1x1 \) -background none \
  -flatten -shave 1x1 -bordercolor black -border 1x1  output.png

That creates a clone of the input image and makes it transparent, uses "-crop 3x4@" to crop it into a grid of 3x4 cells, puts a black border on the top and left edges of the cells, then reassembles them into a grid by flattening them onto the input image. It finishes by adding a border to the right and bottom edges while preserving the input image's original dimensions.

Obviously using this method the cells may not all have the exact same dimensions if the the input image can't be divided evenly by the number of cells.

EDITED AGAIN: If you don't actually need the grid to be an overlay, you can directly crop the image into tiles, put black lines around the tiles, and reassemble them to create the required output image. That can be done with a simple command like this to make 480x360 sized cells...

convert "$infile" -background black -bordercolor black \
   -crop 480x360 -chop 1x1 -splice 1x1 -flatten -shave 1x1 -border 1x1 output.png

Or it can be done with "-crop 3x3@" like this to make a grid of 3 rows and 3 columns letting ImageMagick calculate the sizes as nearly as possible...

convert "$infile" -background black -bordercolor black \
   -crop 3x3@ -chop 1x1 -splice 1x1 -flatten -shave 1x1 -border 1x1 output.png

Again, if the image size isn't evenly divisible by the number of cells, there will be one pixel difference in the sizes of some of the cells.

Mark Setchell · Answer 2 · 2018-02-06T09:18:37.973

I had a little attempt at this and got kind of stymied at every turn. Here is the best I got so far.

convert xc:white[1440x1080\!] -colorspace gray -fx "(i==0||i==479||i==959||i==1439||j==0||j==359||j==719||j==1079)?0:1" grid.png

So, for practical use, you would take your snow scene and clone it. Then go to greyscale (to reduce the processing time by a factor of 3) and set all the lines black and the other pixels white. Then overlay onto the snow scene choosing the darkest pixel at each location:

convert scene.jpg \( +clone -colorspace gray \
   -fx "(i==0||i==479||i==959||i==1439||j==0||j==359||j==719||j==1079)?0:1" \) -compose darken -composite result.png

If you wanted it marginally smarter and adaptive to different sized images, you could calculate the third-points as a function of image size:

magick scene.jpg \( +clone -colorspace gray \
  -fx "(i==0||i==int(w/3)||i==2*int(w/3)||i==w-1||j==0||j==int(h/3)||j==2*int(h/3)||j==h-1)?0:1" \) \
  -compose darken -composite result.png

Nice! I needed a regular grid of size w x h and did it with -fx "(i%w==0||j%h==0)?0:1". Thanks. Helped a lot. — sl0815, Dec 19 '18 at 23:00

score 1 · Answer 3 · answered Feb 06 '18 at 07:07

1

The best I came up with so far:

convert blank.png -fill black -draw 'line 480,0 480,1080 line 960,0 960,1080 line 0,360 1440,360 line 0,720 1440,720' level1.png

But this is all manual.

Is there a way to automate this?

answered Feb 06 '18 at 07:07

theonlygusti

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Several replies and suggestions have been posted here. Some members have put in more than a little effort to help you with your issue. Do you not have any comments about any of them? – GeeMack Feb 08 '18 at 17:14
@GeeMack they were too slow so I did it manually as I wrote in this answer, and haven't needed to do this since. When i next need to do it I will test out their methods, which do seem better, but b/c I am not testing any I don't feel i can accept any – theonlygusti Feb 08 '18 at 17:27
Okay, so your comment, "Is there a way to automate this?," wasn't really a request for replies. The question mark had us fooled. – GeeMack Feb 08 '18 at 17:32
@GeeMack no it is a question. Next time I need to do this task I will be able to do it more easily and faster because of all the helpful answers since my own, and I've also learned a lot about the capabilities of imagemagick. – theonlygusti Feb 08 '18 at 17:34

Mark Setchell · Answer 4 · 2018-02-07T12:48:30.180

Here's another variant that is fast and easy to understand - not 100% identical to your requirements but maybe good enough.

Create a starter box with a black border, duplicate twice and append across the page, duplicate twice and append down the page, make minor correction to size:

convert xc:red[478x358\!] -bordercolor black -border 1  \
  -duplicate 2,0 +smush -1                              \
  -duplicate 2,0 -smush -1 -transparent red -scale 1440x1080\! result.png

score 0 · Answer 5 · answered Feb 07 '18 at 13:10

Or you could make your initial box, duplicate it 8 times and let montage lay the resulting 9 boxes out:

convert xc:red[478x358\!] -bordercolor black -border 1 \
   -transparent red -duplicate 8,0 miff:- | montage -background none -geometry +0+0 miff:- result.png

draw grid with cells of specific size over an image

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