Legacy ImageMagick Discussions Archive

I want to create a gradient with more than two colors and one of the colors should be transparent. Searching the forum I found that -sparse-color in combination with the Shepard method could do the trick. So I tried: The resulting test.png doesn't include the alpha channel. I don't quite understand why. The docs state that one should define -channel and the colors are correct as well as far as I can tell.

On a related note, I'm trying to understand the -fx operator. I have: This means that of the second image (v) the pixelvalue (p{}) is used for the resulting image. But I'm unclear on what pixelvalue is used. The first column obviously, but what row? u is the first image, v the second and h the height? If the first image is greyscale and the second a gradient image as described in my previous post what is the result? I assumed this was a color replacement. A mapping of greyscale to color space. But in that light the multiplier does not make sense to me.

haarts wrote:I want to create a gradient with more than two colors and one of the colors should be transparent. Searching the forum I found that -sparse-color in combination with the Shepard method could do the trick. So I tried:

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convert -size 50x500 xc: -channel RGBA -sparse-color  Shepards '0,0 rgb(0,255,0,1) 0,250 rgb(255,255,255,0) 0,500 rgb(255,0,0,1)' test.png

The resulting test.png doesn't include the alpha channel. I don't quite understand why. The docs state that one should define -channel and the colors are correct as well as far as I can tell.

try adding -alpha set or -matte after -channel RGBA

haarts wrote:On a related note, I'm trying to understand the -fx operator. I have:

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-fx "v.p{0,u*v.h}"

This means that of the second image (v) the pixelvalue (p{}) is used for the resulting image. But I'm unclear on what pixelvalue is used. The first column obviously, but what row? u is the first image, v the second and h the height? If the first image is greyscale and the second a gradient image as described in my previous post what is the result? I assumed this was a color replacement. A mapping of greyscale to color space. But in that light the multiplier does not make sense to me.

v.p{x,y} will look up the pixel at the point x,y in the second image 'v'
The x,y coordinate may be a floating point position so interpolation may be used to mixx colors of near by pixels.

So 0 means just the first 'zeroth' column of pixels will be used,
u*v.h mean the y coodinate is the color of the current pixel in the first image (in 0 to 1 range) multiplied by the height of thesecond image.

So this FX expression replaces the original with a lookup of its color in the second image.
that is replace pixel colors using a Color Lookup Table (the -clut operator is essentuallty does same thing (with minor improvements)

See IM Examples, Color LUT
http://www.imagemagick.org/Usage/color/#clut

anthony wrote:
v.p{x,y} will look up the pixel at the point x,y in the second image 'v'
The x,y coordinate may be a floating point position so interpolation may be used to mixx colors of near by pixels.

So 0 means just the first 'zeroth' column of pixels will be used,
u*v.h mean the y coodinate is the color of the current pixel in the first image (in 0 to 1 range) multiplied by the height of thesecond image.

So this FX expression replaces the original with a lookup of its color in the second image.
that is replace pixel colors using a Color Lookup Table (the -clut operator is essentuallty does same thing (with minor improvements)

See IM Examples, Color LUT
http://www.imagemagick.org/Usage/color/#clut

Now I see! Thanks for clearing this up for me. And I replaced the -fx expression with the -clut expression. Many thanks for the tip.

fmw42 wrote: try adding -alpha set or -matte after -channel RGBA

Adding -alpha resulted in an error and -matte didn't do anything as far as I can see. I resolved the issue by -append and using different gradients. Thanks for the help though!

that should have been "-alpha set"

also to specify transparency in a color you need rgba() nor rgb()
WARNING: shepards currently does not properly handle alpha (sorry)
As such the above will shade the colors from the opaque colro toward transparent white.

Replace the rgba(255,255,255,0) with 'none' to see it shade toward black which makes the problem very visible!

I have added this problem onto my ToDo list.

It is also mishandling -channel A. for example this replaces all existing colors with black!