Just apply a transparent gradient

[DeepShark]

Hello,

I am new with Imagemagick and I don't succeed in apply a gradient on another image.
I have found a lost of topics about this but I haven't succeed in make it work, and honnestly I don't want to study all the doc because it's juste for a one time development.

1 - I create a simple image :
convert -size 500x500 -background blue label:text blue.png

2 - I create a gradient
convert -size 1000x800 gradient:grey-none -function Polynomial -4,4,0 -distort SRT 90 gradient.png

3 - I try to apply the gradient on the image with a transparency effect (in my dreams )
convert blue.png -composite gradient.png output.png

I have also try : convert blue.png ( gradient.png -colorspace Gray -gamma 1.0 ) +matte -compose Copy_Opacity -composite output.png

In fact I have try a lot of things (without really understand them) found in several topics but nothing works.
Any help would be appreciate. Thanks

[snibgo]

Your third command should be something like ...

Code: Select all

convert blue.png gradient.png -compose CopyOpacity -composite output.png

... which will copy the transparency from gradient.png.

However, your images are not the same size, so you might want to adjust.

[anthony]

You may want to include a -alpha off before the -composite when using -compose CopyOpacity.

The original meaning of -compose CopyOpacity is to copy the transparency from source to the destination image. This is the same result you would get using -compose DstIn when the destination image is fully-opaque.

However if no transparency is present, IM treats the second image as a greyscale 'alpha mask' image to assign to the transparency channel of the destination image. Using -alpha off (or +matte ) ensures that the second image at least has no transparency channel to be copied.

Without that addition you may find the composition doing nothing (coping fully-opaque transparency channel) for no visible reason in some cases.

See the warning in
http://www.imagemagick.org/Usage/compose/#copyopacity


Recent Development: a new addition to IM is a setting ( -intensity ) that can change how the compose CopyOpacity (and CopyBlack) methods reads a 'grayscale intensity' from the given source/overlay image, especially from more 'colorful' images that have no alpha channel. When the composition method is to be updated to use this setting is unknown.

[DeepShark]

Thanks for your answer, it make me progress a lot

The solution of snigbo was quite good for me (the -alpha off from Anthony doesn't work for me, no matter).

But I know realize it's not exactly what I need for my effect.
Because what I want is to make a part of my image "darker" on one side (it's for a shadow on a object).

What I want with my gradient (which is from white to grey) is : nothing change on my object where the gradient is white and a bit darker where the gradient is grey.
But in fact, currently the result is different : I see the original image where the gradient is grey and I see a white zone where the gradient is white.

Do you see what I mean ? Any idea for that ?

Thanks

[snibgo]

What I want with my gradient (which is from white to grey)...

It isn't. You are probably using a viewer that shows transparent pixels as white. In your gradient command, "none" means "transparent black".

I'm confused about what you want. Do you want any transparency in the output?

[DeepShark]

Yes, I think you're right, it should be transparent. But my viewer don't show transparency, so I can't know it's transparent ou white.

What I want to make is somthing like this : http://tavmjong.free.fr/INKSCAPE/MANUAL ... Can_14.png
(don't look at the page, it doesn't matter, I just take the image in google image to show the result I want).

I have already made the sticker, which is plane currently.
Then I want to make the shadow on it.
Because I am going to apply it on a background which is a bottle, with a shadow on it too.
Si I want the sticker to have the same kind of shadow.
Is it more clear ?

So my sticker is white with some black/red letters, and now I want (with the gradient) make a part of it darker to simulate the shadow.