Experimenting with different brushes and effects:


This is fun and perhaps addictive

Not tested but after a brief look, I think this could be reproduced in 2.6 with two brushstrokes:

- First brushtroke with a hard edge brush set up the gradient (black-blue to mid-grey-blue) and fill the path 100%.
- Second brushtroke uses a high-grey-blue starting from the top with fade in activated and uses a soft edge brush which will shade the sides so they feel 3Dimensional.

Well, it's not that simple as it may seem. If you watched the video from the second link in my first post, it's a presentation of a "new, exciting feature"
in CS6 Adobe Illustrator (also referred to as a "real magic" in that video). And CS6 is packed with quite sophisticated software. The reproduction method you've described was possible in Photoshop 7, the predecessor of CS series. Applying gradient along, and especially across, the path in complex objects like spirals in that video is something that can't be easily achieved in GIMP 2 or even in Inkscape (the vector graphics editor similar to AI). YAFU and Esper suggested a workaround based on mahvin's technique and RobA's path blend script. You can get close results with those but only to reproduce a directional gradient along the path with more or less success. Making a gradient stroke across the length of a spiral is still a 'magic' for many image editors.

Another way to fake irregular gradient - interpolate path

interpolate path ???????

this was made with interpolate paths in inkscape. In GIMP there is inteprolate paths plugin (Generate intermediate paths by Ofnuts) but I haven't tried it to reproduce this effect. Don't remember if it can interpolate styles (colors in gimp terms)

I've just tried Ofnuts' script to generate intermediate paths. I created two identical curved paths, moved them apart at 90 pxls and ran the script set to 50 total steps. 50 additional paths were created, but not between start and end paths, not offset. Probably 50 is too much for the script or there was something I didn't do right. But I am sure it's possible to write a script which would create intermediate paths between defined start- and end-point and stroke each path with colors changing according to chosen gradient. Thus this irregular gradient effect could be reproduced in GIMP. Hope Ofnuts reads this thread, he's one of the best paths script writers out there.

Something like this could be achieved with such a script as well

So I am to understand from the preceding messages that there is no way to easily create a gradient along or across a stroke using either GIMP or Inkscape... Very disappointing. I am also finding it impossible to produce such an item within an SVG file using Illustrator CS6 either: the stroke looks fine in Illustrator, but if I convert the file to an SVG and then open it in Internet Explorer or Firefox, my stroke appears to have been converted into a blocky raster instead. Is there no graphics program capable of producing a stroke vector with a gradient that goes along or across it?? Is this really such a mysterious thing? I am baffled that it appears so baffling. Someone please tell me I am wrong...

Here is an example of a file with which I have been having this difficulty:

upload.
wikimedia.
org/
wikipedia/
commons/
archive/3/32/
20130425093914!Saltwater_Limpet_Diagram-en.svg

(Put that all on one line— the gimpchat machine would not let me post the URL properly, calling it "spammy").

Note the yellow "nerve cords" on the far left, hooking around the animal's head: these are strokes with gradients across them, and they have always come out looking like they've been through a blender. I was hoping to find some other graphics program— ANY program— that could do what AI pretends to do with this kind of object (in AI, the image looks just fine!).

Having said this, I just attempted to create a simple image of a single stroke with a gradient across it using AI, and the SVG version of it, well... it looks like it turned out looking ok.

So now I am super stumped.

Here's another image, this time of just the intestines, converted into PNG format (GIMP would not let me upload SVGs). Again, you can see the jagged edges (yes?). This is another stroke with a gradient across it which looks fine in AI and gets clobbered when AI converts it to an SVG image:


(But can you even see it up close enough here to tell??)

Here is another copy of the same image with a straight stroke that has a radial gradient along side the squiggly intestine. Can you see the difference now?



Lastly, here is how the green intestines appear— more or less— in Adobe Illustrator: clean, crisp (but here rendered as a bitmap and not a vector... which is exactly the problem):


(and then, just to make the confusion complete, look at the difference between the intestines as rendered in the first image by itself and then in the second image with the straight line: the one giving me the problems looks WORSE in the second image, and the only difference between them is the addition of the straight line! WTF??)

Thank you for taking such a thorough look at this. What you are seeing is exactly what I am experiencing: a raster image where a vector should be. I will gladly let you look at the .ai version of the file, but GIMP won't let me upload it here and Wikimedia forbids it as well— any suggestions?? I have gmail and yahoo accounts-- could I make the file available through one of them? Or should I send it to you directly?

You are correct:

GIMP will generate a gradient transverse to the path if you copy the gradient to use as your brush and then use stroke with brush dynamics set to "track direction" but because this applies multiple stamps the result can be quite rough and ugly in places (reduce spacing, use a thin brush, soften brush edge, create enlarged version of path and scale down)

alternatively use "path-to-selection" to colour interior and exterior regions in a fresh layer, then blur to turn that into a a gradient aligned with the path and then apply a mask from the border of that region to emulate the path .. clunky but this sort of method is often used to get soft shadows under text

here is an example using both those techniques

I can get this using stoke path emulating dynamics with Airbrush and using that circle you can see in the image as a brush, from the clipboard. Brush spacing = 1:

If you use the paintbrush then appear jagged edges.

But as you want a vector graphic I do not know if there is any program that can do the conversion from bitmap with good quality.

I think GIMP developers could make a very powerful tool for stroke along path. For example for the crossings they could make GIMP detect when a stroke must pass in front or behind. Maybe that could be done by marking paths in two layers, top layers for front strokes and bottom layer for strokes that must pass behind. Also should improve jagged edges when using paintbrush.