Legacy ImageMagick Discussions Archive

Hello,

I've been trying to figure out the best way to add a selective transparency to an image.

Let's say I have 1280x720 image and I want to apply a 50% transparency over the last 100 bottom pixels (with 100% width).

Could this be done without creating a mask first? I haven't managed to figure out a decent solution.

I suppose you could use "-fx" but that would be very slow. I think you need a mask of some sort, for example (Windows syntax):

Code: Select all

convert ^
  in.png ^
  ( -size 1280x720 xc:white -fill gray(50%%) -draw "rectangle 0,620 1279,719" ) ^
  -alpha off -compose Copy Opacity -composite ^
  out.png

If you are doing this many times (eg subtitles for video), it may be faster to create a file once, then use that for each frame:

Code: Select all

convert ^
  -size 1280x720 xc:white -fill gray(50%%) -draw "rectangle 0,620 1279,719" ^
  mask.mpc

convert ^
  inframe.png ^
  mask.mpc ^
  -alpha off -compose Copy Opacity -composite ^
  outframe.png

I think you have to create a mask first, but the whole thing can be done in one command line using parenthesis processing. see http://www.imagemagick.org/Usage/basics/#parenthesis

convert image \
\( -clone 0 -fill white -colorize -size 1280x100 xc:gray50 -gravity south -compose over -composite \) \
-alpha off -compose copy_opacity -composite result.png

On second thought, here is a way that does not need a mask

Code: Select all

convert image -alpha set -channel rgba -region 1280x100+0+621 -channel a -evaluate set 50% +channel show:

That's awesome! I knew it could be done!

Thanks

Hmmm, I seem to be running into a weird issue when using some already processed images. I'm doing this:

convert.exe: geometry does not contain image `temp\canvas.png' @ warning/transform.c/CropImage/666.

Code: Select all

tools\convert.exe -background #181818 file.jpg -resize 1280x1280^> -gravity center -extent 1280x720 -font Calibri -pointsize 14 -gravity north-east -stroke #000C -undercolor #00000022 -strokewidth 3 -annotate 0 " made with stics - http://linge-ma.ws/stics " -stroke none -fill white -annotate 0 " made with stics - http://linge-ma.ws/stics " temp\background.png

Then:

Code: Select all

tools\convert.exe temp\background.png -font ./tools/impact.ttf -pointsize 42 -gravity south-east -page -30-130 -stroke #000C -strokewidth 3 -annotate 0 "BTSTU (JAI PAUL COVER)" -stroke none -fill white -annotate 0 "BTSTU (JAI PAUL COVER)" -page -30-172 -stroke #000C -strokewidth 3 -annotate 0 "NIIA" -stroke none -fill white -annotate 0 "NIIA" temp\canvas.png

When I'm doing the transparency part, I get this:

Code: Select all

tools\convert.exe temp\canvas.png -alpha set -channel rgba -region 1280x100+0+621 -channel a -evaluate set 50% temp\canvas.png
convert.exe: geometry does not contain image `temp\canvas.png' @ warning/transform.c/CropImage/666.

What am I missing here?

If I use temp\background.png for the transparency, the last command works. What's going on there?

tools\convert.exe temp\canvas.png -alpha set -channel rgba -region 1280x100+0+621 -channel a -evaluate set 50% temp\canvas.png

Syntax is different on Windows. You need to use %% rather than %. See http://www.imagemagick.org/Usage/windows/

It is always a good idea to identify your version of IM and platform when asking a question.

Also you have to change the region for each different image size. Check the size of your input image canvas.png

I'm using ImageMagick 6.8.8-3 Q16 x64 2014-01-19 on Windows.

The % doesn't seem to be the issue, as, like I said, it works fine if I use background.png instead of canvas.png

My image size never changes, it's always 1280x720.

Something is going wrong when I'm annotating the image (the 2nd step) and I can't seem to figure out what...

-page -30-130

This may be the issue. It appears to be shifting the image outside the bounds. Check its verbose information and see if Page geometry: has offsets and different size.

Otherwise, just try using +repage before saving the image.

Also the parens in your text may have to be escaped. But I do not use Windows and so am not sure about that.

That didn't work out either

I do not see +repage anywhere. Am I missing it?

Did you get the verbose information

identify -verbose yourimage

and check the Page Geometry?

Can you post a link to the input image for the transparency command? I can then try that image with my command.

Canvas.png:

Background.png:

I used +repage at the end of the annotate command (+repage temp\canvas.png).

Geometry on both files say 1280x720

Is temp your directory or the system directory. If the latter, it is not wise to do processing there. You could be running out of space.

It's my own directory.

Your canvas image still has an offset. Its Page geometry: 1280x720-30-172. So your +repage either was not there or did not work, which is strange.

But this works for me

convert canvas.png +repage -alpha set -channel rgba -region 1280x100+0+621 -channel a -evaluate set 50% +channel show:

What is the purpose of -page -30-172 in your earlier command?

If you want to offset the annotation, -annotate has offset arguments. See
http://www.imagemagick.org/script/comma ... p#annotate
http://www.imagemagick.org/Usage/text/#annotate

Legacy ImageMagick Discussions Archive

How to apply a selective transparency to an image?

How to apply a selective transparency to an image?

Re: How to apply a selective transparency to an image?

Re: How to apply a selective transparency to an image?

Re: How to apply a selective transparency to an image?

Re: How to apply a selective transparency to an image?

Re: How to apply a selective transparency to an image?

Re: How to apply a selective transparency to an image?

Re: How to apply a selective transparency to an image?

Re: How to apply a selective transparency to an image?

Re: How to apply a selective transparency to an image?

Re: How to apply a selective transparency to an image?

Re: How to apply a selective transparency to an image?

Re: How to apply a selective transparency to an image?

Re: How to apply a selective transparency to an image?

Re: How to apply a selective transparency to an image?