Need to map pixels: how to speed up txt: output ?

[homer314]

Hi all

i need to create an edges pixel map using some BIG images and the txt: output takes a lot of time to write the file.

What i'm doing now is to convert the image with

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convert in.png -alpha extract -edge 1 out.png

and then

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convert out.png txt: | ... 

with the varius grep and sed arguments to extract white pixels coordinates, but commands must to read hundreds of input lines every time, causing slow elaboration.

So is there a way for a FAST edges-only pixel mapping on BIG images ?

Really thank's

[fmw42]

what is your exact command using grep and sed, etc.?

You might be able to convert much of that into using AWK, which may be faster.

[homer314]

what is your exact command using grep and sed, etc.?

You might be able to convert much of that into using AWK, which may be faster.

Thank's for answer, i use

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convert $f txt: | sed 's/ *,/,/g' | grep "white" | cut -d: -f1 

to extract white pixels coordinates.

There's no way to tell to IM txt output to work only on white pixel directly, without creating a very large number of line input for grep, sed, or awk?

Thank's

[fmw42]

You may be able to speed it up a bit by putting the grep "white" first. You can also speed it up by combining the sed and cut commands into an sed search and replace.

try this

convert rose: txt: | grep "white" | sed -n 's/^\(.*\):.*$/\1/p'

[homer314]

Thank's, just another little question:

i use "-alpha extract -edge 1" (tried also higher edge values) to create the black and white png to use to extact pixel coordinates and all is working fine expect in a case.

If i've a circle selection, edges ar cutted out on borders, like in the example




It is possibile to prevent this ?

Thank's

[fmw42]

try padding the image with whatever is the background color

find the background color before edges are extracted, I presume black for now.


convert in.png -alpha extract -gravity center -bordercolor black -border 2 -edge 1 out.png

[anthony]

The edge dector will generall get either the inside edge, or the outside edge, depending on which is lighter or darker. Looks like you detected the outside of the edge.

If you negate the image before doing the edge effect the edge will go in the oppisite direction.

See IM Examples, transforms, Edge
http://www.imagemagick.org/Usage/transform/#edge

Alternative, just draw the circle using -fill none -stroke {color}

If you need it absolutely balck and white (bitmap), then you need to threshold the image to remove 'grays'.

Give the command you are using and we can help further!

[homer314]

ok, these are my steps:

starting from this image for example (imageshack upload remove my alpha channel but you consider it)



i need to find the edge's pixels coordinates so:

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convert $im_in -resize "$weight_dpi"x"$height_dpi" \ #I RESIZE THE IMAGE TO 96DPI TO SPEED UP THE TXT OUTPUT EXTRACTION
        -alpha extract -edge 1 \ #THIS CREATES THE IMAGE IN THE FIRST  POST
        -crop 1x"$lines"@ +repage +adjoin tmp_%d.png; #I DON'T NEED ALL EDGES PIXELS, BUT JUST THE MAX AND MIN "X" FOR EVERY OF THESE SLICES

...then i use txt: output on every slice

The code above works fine with all the tests images i tried, except when there is an image with "non closing edges"

Really sorry for my bad english, and really realy thank's for help

[anthony]

You could just take the image then using -trim.
By reading the resulting virtual canvas information you will get you X and Y 'bounds'.

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convert image.png -trim -format -format 'MinY = %X  MinY = %Y MaxX = %X+%w-1 MaxY = %Y+%h-1 ' info:

You can use %[fx:...] formulas to do the calculation too. Or just output the 'crop' that -trim applied which is simply...

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convert image.png -trim -format -format '%wx%h%O' info:

[homer314]

Really really thanks for all of these helpful tips