Hi,

I'm currently trying to create a complex colour gradient out of a number of primitive grayscale gradients whose values are mathematically significant. I probably ought to be learning a plotting program like GNUPlot or R for this, but I was really hoping that I could get away with doing it in SVG, partly because the result is ultimately going to be used in an Inkscape drawing, and partly because those programs frighten me I've got as far as creating a combined grayscale gradient that represents what I want it to by using a bunch of filter operations on a few overlapping underlying gradients. What's stumping me slightly is that I'd now like to map the relatively narrow grayscale gradient I've ended up with to a range of hue values at maximum saturation and mid-lightness. In HSL terms, effectively I want to map the lightness of the grayscale to a hue value. Since I want to do this non-destructively based on the underlying primitives, I've been looking at filters to achieve this, but I'm a bit stumped by the fact that they all work in RGBA. I guess the crude way to do it would be a table-based component transfer, but that's pretty fragile. Does anyone with more of a head for maths/filters have any clever ideas?


Many thanks,

Lucian

Hi.

As far as I know, inkscape/svg works in the rgba space, even though you can "hack" use pre-installed icc profiles (?).
Filters are raster based. If you want to save the image as a pdf later on, you will have some limitation on them.
Because of that, I wouldn't even suggest to use masking, it doesn't work either.

So something that might work, is to edit the greyscale gradient to black to black&transparent, white to white&transparent parts.
Though this would work best if the midtone below was really mid-tone, with lightness value at 127,5.; some small compromise is necessary.

I'm not 100% sure I get what you are suggesting. My "grayscale gradient" was in fact already created by overlaying multiple gradients that faded from alpha 1 -> alpha 0. But the final step represents the addition of two overlapping sources of radiation, and I want this to be a "pure addition" operation (modelled using addition Composite filter- K2 &K3 both set to 1). This part I haven't had difficulty with. It was the translation to a range of colours that I was stuck on - because obviously the relationship between hue value and RGB values is a complex one that cannot (AFAICT) be modelled as an equation simple enough to be expressed in inkscape filters. In the end I got an approximation using a Table-based ComponentTransfer with hand-tuned values. It looked terrible because of the multiple-levels of raster processing, but it gave me something on which to base a hand-drawn replacement using a bunch of filled concentric paths that I then blurred together to look like a color gradient. The nerd in me would have liked to have had the final version working direct from the underlying data; but on the plus side it's now a great deal faster!

FWIW I have no need to export to any other vector formats - it's a map that will ultimately be rendered either to screen, bitmap or print.

Thanks for the suggestion anyway.

Lucian

I was thinking of something similar to this.

Hey, thanks for replying again - I fear that I'm not really explaining myself very well. The final output was supposed to be something like your bottom gradient - full saturation all the way across - but I wanted to *generate* this by transforming an underlying grayscale or alpha gradient that describes where and how quickly the transitions between the colours should occur. This was the manual output I produced in the end:



Those colours represent climate bands. The cosmology of the world being mapped is a bit odd - a disc with two small, closely orbiting suns which pass over the east and west ends. I produced a spreadsheet that listed out the levels of insolation at given distances from each solar path based on the sizes of the suns, the angle to the surface, atmospheric absorption etc.

I then produced two sets of gradients that represented distance from either solar path, and applied a Table ComponentTransfer filter to map this distance to the insolation values from my spreadsheet. Finally, I positioned the gradients based on the solar paths, which resulted in an overlap; I applied an Arithmetic Composite filter to the upper gradient to perform a straight addition of the values in the gradients. This produced a final grayscale gradient where the luminance value at any given point represented the total level of solar insolation there. What I was then hoping to do was to translate that into a set of colours ranging from blue (lowest value) to red (highest value) to produce an image that looked a bit like the one above. The only way I could find to do that was to use another Table ComponentTransfer and try and model the relationship between hue and RGB manually that way. I got a result that had roughly the right colours, but it was really pixelated because of all the raster transformations, so I just traced over it manually to produce the image above. A lot easier!

Cheers,

Lucian

Lazur URH wrote:I was thinking of something similar to this.

By the way, I forgot to ask, what did you use to create that colour wheel gradient? I looked through the extensions but couldn't find anything that obviously would do that - or did you generate the SVG outside of inkscape?

Thanks,

Lucian

That gradient was made with inkscape manually -each step was snapped to guidelines at even distances.

Now that I see what you wanted to achieve, a "gradient map" effect can do similar with rasters.
Available in gimp as a plugin only (?) http://docs.gimp.org/en/plug-in-gradmap.html, while in photoshop it seems a built in feature.

Nice image by the way!

Lazur URH wrote:Available in gimp as a plugin only (?) http://docs.gimp.org/en/plug-in-gradmap.html

Built in to my Gimp 2.8 - Colours > Map > Gradient Map (make sure to select your target gradient first from the Gradients dialog (CTRL-G)). It used to be in the filters menu in older releases, I'm not sure when it moved.

Thank's for the correction!

Lazur URH wrote:That gradient was made with inkscape manually -each step was snapped to guidelines at even distances.

Wow. That must have taken a while!

Lazur URH wrote:Now that I see what you wanted to achieve, a "gradient map" effect can do similar with rasters.
Available in gimp as a plugin only (?) http://docs.gimp.org/en/plug-in-gradmap.html, while in photoshop it seems a built in feature.

Yes - that's *exactly* what I was looking for. I'm beginning to see why most people do this sort of work in GIMP/Photoshop rather than Inkscape. On the other hand I much prefer working with geometry and mathematical operations than pixels and brushes - I'm much more of a geek than I am an artist I guess!

Lazur URH wrote:Nice image by the way!

Thanks! It's very crude by comparison to some of the dauntingly beautiful work I've seen on Cartographer's Guild but I guess we all have to start somewhere! Now I just have to fill it all in with some actual geography... spent most of yesterday working out how to build mountains out of lighting filters; and most of today working out and writing up the Inkscape bugs I found yesterday Hopefully at some point I'll have something I don't feel too embarrassed to share on one of the forums here....