Center image by extending only one dimension

[jamadagni]

I would like to know how to tell IM to extend only one dimension (X or Y as per my wish), center the image in it and fill the rest with background colour. For example:

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convert -size 1000x1000 xc:white rose: -gravity Center -composite out.png

...will certainly center the rose in the middle of the 1000x1000 white field. But say I want to specify that the rose should be centered in a white field of:

1. 1000 px width and height same as rose, or
2. width same as rose and 1000 px height

then how do I do it? Preferably the solution should work with other gravity options such as N, S, E, W too (though the NE, NW etc would be not meaningful). Thanks!

[fmw42]

If you want to shift the position relative to the gravity, then add -geometry +Xoffset+Yoffset after -gravity .

You can also just use -extent. See viewtopic.php?f=1&t=33012

[GeeMack]

But say I want to specify that the rose should be centered in a white field of:

1. 1000 px width and height same as rose, or
2. width same as rose and 1000 px height

then how do I do it?

As fmw42 mentioned, you can use "-extent" in a fairly simple command like this...

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convert rose: -background white -gravity center -extent 1000x0 out.png

That would create your first example with the rose centered in a white canvas of 1000 pixels wide and the height of the rose. Using zero in that geometry argument tells "-extent" to just use that dimension from the input image. Of course you'd swap the width and height arguments to make the canvas 1000 pixels high and the width of the rose.

[fmw42]

You actually do not need the 0 for height. IM will take the full image height. So you can just do

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convert rose: -background white -gravity center -extent 1000x out.png

[jamadagni]

☺ to both of you!