convert: extract an horizontal band

[Cequiel]

Hi Everybody,
I have the following command line:

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convert \( some drawing, scaling, rotation and translation operations... \) output.png

The above command is the result of apply several image operations. The result of these operations is an arbitrary image and the (0, 0) point is not necessarily at the top left corner. I would like to extract an horizontal band and I tried this command:

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convert \( some pictures, drawing, scaling, rotation and translation operations... \) -extent 0x125 output.png

But the above command doesn't work properly because the 'extent' directive assumes that the (0, 0) point is at the top left corner (which is not always true). How could I extract an horizontal band of an arbitrary image with arbitrary geometry starting from the (0, 0) point?

Here's an illustration showing what I would like to do:


Thanks and sorry for my English

[snibgo]

The usual operation for extracting a portion of an image is "-crop". This takes any canvas into account. Generally, follow "-crop" with "+repage" to remove canvas information. For example, on Windows, I make an image g.png where (0,0) isn't top-left. (0,0) is close to bottom-right. Then I crop, and the result g1.png comes from the bottom-right of the input. I asked for 100 pixels width, but the result is only 20 pixels wide, because that is all the pixels to the right of (0,0).

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F:\web\im>%IM%convert -size 100x100 gradient: -repage -80-80 g.png

F:\web\im>%IM%identify g.png
g.png PNG 100x100 100x100-80-80 16-bit sRGB 647B 0.000u 0:00.000

F:\web\im>%IM%convert g.png -crop 100x10+0+0 +repage g1.png

F:\web\im>%IM%identify g1.png
g1.png PNG 20x10 20x10+0+0 16-bit sRGB 286B 0.000u 0:00.000

If this doesn't help, please post an input image, and say what portion you want to extract.

[snibgo]

I can use your image as an example. I move (0,0) to where you want it:

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convert example2.png -repage -187-93 e.png

I check this has worked:

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identify e.png

e.png PNG 563x317 563x317-187-93 8-bit sRGB 128KB 0.000u 0:00.000

The steps above would be done by your rotation, translation, etc.

I crop it to 125 pixels high:

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convert e.png -crop x125+0+0 +repage cropped.png

cropped.png is:

[Cequiel]

Thanks snibgo. The problem is that I don't know where is exactly that point. Actually, the image is generated automatically by a mysterious algorithm

I can use your image as an example. I move (0,0) to where you want it...

[Bonzo]

So you have a point that you do not know where it is and you do not know how to find it and you want to use it as the reference

I would think you need to post a real life image for people to try.

I think snibgo is saying that the point you want may be in you image file but not in the image file you posted here so he had to manually create it. Anyway as I say best to post a real life example.

[Cequiel]

I didn't know the 'identify' command. It was very useful. I did the following:

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// creates the image
convert \( mysterious algorithm ... \) output.png

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// gets the offset Y
identify -format '%Y' output.png

// the above command returns
+4294967207

Actually, the offset "4294967207" is an unsigned value. If you want to get the signed value, you have to do the following:

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<?php
$base = pow(2, 31);
if ($offsetY > $base) $offsetY -= 2 * $base;

In this case the unsigned value "4294967207" is equal to the signed value "-89". Now, you can mount the command to extract the horizontal band:

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// change '-89' by '+89'
convert output.png -background transparent -extent 0x125+0+89 +repage output.png

The above command extracts a band starting from the point (0,0). It is very complicated and very hard to implement, but it works.

The usual operation for extracting a portion of an image is "-crop". This takes any canvas into account. Generally, follow "-crop" with "+repage" to remove canvas information. For example, on Windows, I make an image g.png where (0,0) isn't top-left. (0,0) is close to bottom-right. Then I crop, and the result g1.png comes from the bottom-right of the input. I asked for 100 pixels width, but the result is only 20 pixels wide, because that is all the pixels to the right of (0,0).

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F:\web\im>%IM%convert -size 100x100 gradient: -repage -80-80 g.png

F:\web\im>%IM%identify g.png
g.png PNG 100x100 100x100-80-80 16-bit sRGB 647B 0.000u 0:00.000

F:\web\im>%IM%convert g.png -crop 100x10+0+0 +repage g1.png

F:\web\im>%IM%identify g1.png
g1.png PNG 20x10 20x10+0+0 16-bit sRGB 286B 0.000u 0:00.000

If this doesn't help, please post an input image, and say what portion you want to extract.

[Cequiel]

I think there should be an argument similar to 'extent', but respecting the geometry. It would be very useful.

[fmw42]

I think there should be an argument similar to 'extent', but respecting the geometry. It would be very useful.

More properly, you should be using -crop and not -extent. However, -extent does crop as well as extend.

With regard to cropping relative the virtual-canvas, I think IM already does that. see http://www.imagemagick.org/Usage/crop/#crop_page. Note that if the offsets are positive, this means the origin is left and above the top left corner. For origins inside the image, the offsets will be negative. (I just tested this to be sure)

But if I read your post above, your origin is not specified as a proper offset in the virtual-canvas. Or did I misunderstand why you had to do all the calculations.

see
http://www.imagemagick.org/Usage/crop/#crop
http://www.imagemagick.org/Usage/crop/#extent