how to do a transparent gradient?

[zomp]

In my last thread I asked a suggestion to draw a 3D cylinder. Bonzo, that I thank, has stimulated me to study the draw syntax. My solution is:

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convert -size 120x60 xc:none -fill none -stroke black -draw "roundrectangle 5,5,115,55 25,25" -draw "roundrectangle 5,5,55,55 25,25" cyl1.png

The result is ok for me.
Now, since I want to make the cylinder transparent, I need some type of gradient where one color is none (that is, transparent).

Unfortunately, it seems to me that I have not well understood how to use the gradient: option.

I tried without success the following command:

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convert cyl1.png gradient:none-white -flatten gradient.png
convert: Must specify image size `none-white'

and this other one:

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convert cyl1.png -fx 'cos(pi*(i/w-0.5))' -flatten gradient.png
convert: unable to parse expression `cos(pi*(i/w-0.5))'

(I work on a Windows XP machine, sorry)
Let me say that I copied the -fx argument from an example in page http://www.imagemagick.org/Usage/canvas ... ent_resize.
Even if I type the command exactly as it is reported in the example,

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convert  rose:  -fx 'cos(pi*(i/w-.5))'   gradient_fx_cos.gif

I obtain the same error message.

Furthermore, this example draws a horizontal symmetric gradient, even if I need a vertical one. Since it is not clear for me the meaning of i and w, I am not able to modify the formula inside parenthesis to obtain the effect I want.

Thanks for your help and your patience.

Your program is wonderful.

[el_supremo]

You need to use the double quote character around the cos function. "cos(pi*(i/w-0.5))"

Pete

[el_supremo]

convert cyl1.png gradient:none-white -flatten gradient.png
convert: Must specify image size `none-white'

I tried that command using the logo: image (since I don't have your cyl1.png image) and it fails with the same message but:

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convert logo: -size 640x480 gradient:none-white -flatten gradient.png

Seems to work OK using ImageMagick 6.3.5 07/31/07 Q8 on Win SP Pro sp2.

Pete

[zomp]

convert cyl1.png gradient:none-white -flatten gradient.png
convert: Must specify image size `none-white'

I tried that command using the logo: image (since I don't have your cyl1.png image) and it fails with the same message but:

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convert logo: -size 640x480 gradient:none-white -flatten gradient.png

Seems to work OK using ImageMagick 6.3.5 07/31/07 Q8 on Win SP Pro sp2.

Pete

Thanks for your help.
In your opinion, is it possible (and if yes, how) to obtain a gradient from [none] to [white50%-none50%] ?

[zomp]

You need to use the double quote character around the cos function. "cos(pi*(i/w-0.5))"

Pete

Thanks. It works even without quotes, as I have succeeded yesterday at very very late hour after many experiments. I do not know if that depends on my OS, but I write here hoping this can be useful to someone in the future.

[Bonzo]

The gradiant none has not been around long and so you need a current version to use it.

I created my gradiant and then rotated it through 90 degrees before using it.

[zomp]

Furthermore, this example draws a horizontal symmetric gradient, even if I need a vertical one. Since it is not clear for me the meaning of i and w, I am not able to modify the formula inside parenthesis to obtain the effect I want.

Solved. I have to use j and h in place of i and w.

Now the only one question to answer is: is it possible to make a gradient from none to a half-transparent color?

[el_supremo]

You can use the rgba function to specify a partially transparent colour like this:

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convert -size  256x256 gradient:none-rgba(100%,0,0,0.5) gradient.png

Note that the Alpha component is a fractional value from zero to one.

Pete

[zomp]

You can use the rgba function to specify a partially transparent colour like this:

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convert -size  256x256 gradient:none-rgba(100%,0,0,0.5) gradient.png

Note that the Alpha component is a fractional value from zero to one.

Pete

Unfortunately, all my trials to work with -fx and -flatten options had no success!

For example, the following command:

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convert -size 120x60 -fx cos(pi*(j/h-0.5)) gradient:none-white  gradient.png

draws a gradient from N to S but not centered to the half height of the image, and from none to black instead of from none to white!

Similar result for various trials with -flatten option.

Maybe I have choosen a roundabout way.

Can you suggest me a clear command to obtain a png image with gradient varying from trasparency at the median height to some color at the top and the bottom of the image?

Thanks for your patience.

[Bonzo]

I see what you mean about the gradiant it seems to be more transparent than colour.

All I can think of is make the gradiant image larger and crop it.

[zomp]

I see what you mean about the gradiant it seems to be more transparent than colour.

All I can think of is make the gradiant image larger and crop it.

Yes, this is a smart suggestion.

But I have great problem first: I am not able to use -fx option to yield a gradient from some color to none or viceversa. All the temptatives I have made yield an image with black-white gradient. I have read that -fx option accepts also colour codes, but I have not found an example useful to adapt to my case.

[anthony]

One work -flatten will always start with an image that is the current background color. That is usally an opaque color, so when your image is -flatten'ed you will lose all transparency.

Set -background none first if using -flatten and want to preserve transparency. -flatten with one image and a background of 'none' is only useful when wanting to 'fill out' a virtual canvas which I don't think you are doing.

It is normally a layering operator. not a gradient handling operator. But that 'background' part makes it useful for removing transparency.