using draw with alpha

[jox81]

Hello,

I'm trying to find a "singleline" command to draw a rectangle in the alpha channel.

This work in the rgb channel :

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-alpha opaque -fill 'rgba( 0, 0, 0 , 0.5 )' -draw 'rectangle 66,50 200,150' 

This work in the alpha channel :

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-alpha opaque -fill 'rgba( 0, 0, 0 , 0.5 )' -draw 'alpha 4,4 point' 

I don't know if this is possible, but how can i draw a rectangle directly in the alpha channel ?

Thanks in advance

[snibgo]

What version IM do you use? On what platform?

[jox81]

Sorry for the lack of informations.

I'm on windows 10 with ImageMagick-7.0.5-9-Q16-x64.

[fmw42]

Using the IM internal image logo:. You can replace that with your image of the form image.suffix

Windows syntax:

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magick logo: -alpha set ^
( +clone -alpha extract -fill black -draw "translate 320,240 rectangle -100,-100 100,100" ) ^
-alpha off -compose copy_opacity -composite result.png

If using Windows 10 unix, then

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magick logo: -alpha set \
\( +clone -alpha extract -fill black -draw "translate 320,240 rectangle -100,-100 100,100" \) \
-alpha off -compose copy_opacity -composite result.png

[snibgo]

To draw with transparent fill, the image you are drawing on should be fully transparent. For example (Windows BAT syntax):

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%IMG7%magick ^
  -size 600x400 xc: -alpha transparent ^
  -fill rgba(75%%,100%%,0,0.35) ^
  -draw "rectangle 66,50 200,150" ^
  -draw "rectangle 150,100 250,300" ^
  out.png

If you want to draw with transparent fill over an opaque image: first clone the image, make it transparent, draw on it, and composite this over the opaque image.

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%IMG7%magick ^
  logo: ^
  ( +clone -alpha transparent ^
    -fill rgba(75%%,100%%,0,0.35) ^
    -draw "rectangle 66,50 200,150" ^
    -draw "rectangle 150,100 250,300" ^
  ) ^
  -compose Over -composite ^
  transdraw.png

Does that answer the question?

EDIT: An alternative is to use "-background None":

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%IMG7%magick ^
  logo: ^
  -background none ^
  -fill rgba(75%%,100%%,0,0.35) ^
  -draw "rectangle 66,50 200,150" ^
  -draw "rectangle 150,100 250,300" ^
  transdraw2.png