Hello everyone,
I’m trying to understand how to create a linear lens flare effect in Inkscape, something like a thin light flare/reflection in a straight line.

I tried searching on Google and online, but I could not find anything really useful, no clear tutorials and no practical advice specifically for Inkscape.

I would like to understand if:

• there is a method or trick to do this in Inkscape;
• there are tutorials, examples, or sample files to study;
• or if this effect is difficult to make because Inkscape is missing some specific feature.

Even a simple suggestion would be helpful, for example:
blur, gradients, filters, masks, blend modes, duplicates, or any other method.
example image

Thank you.

Hi there, 

many of your suggested approaches could work. 

So basically the example image shows a pointy gradient which transitions between a saturated colour and white towards the middle, 

and also between semi-transparent and transparent. 

Can blurring work? The pointiness is the bottleneck. It spreads out the boundaries of an area, making it impossible to use it on a pointy base shape.

Instead, it could be used to generate a half/quarter part of the gradient, then clip it to a shape, and by using cloning you can build up the full image.

Masks? That could also be helpful to separate the transition between opaque steps and a transition on the alpha channel.  

Then you'd need to draw two colour transitions, one from white to a vivid colour, then another from white to black. 

Group each together and mask the two groups with one another.

Actually there is a bit more to than that, because to avoid the gap issue between copies you'd need two full shapes beforehand for the masking. 

(And not mirroring 4 copies of a masked quarter.)

Instead of using blurring, there is the option to use interpolation between paths and interpolate between styles.

Then you'd definitely need to use masking.

Another possible solution couldbe by using mesh gradients. 

With filters you can change the hue value of the already existing flare. There is no "bloom" filter primitive, only Gaussian blur so they have a limited use here as per se.

Blend modes are also filters, using the background image as an input (if you are referring to layer blending modes).

They are especially resource heavy, so better to use the blend filter primitive inside a custom filter to adjust the appearance, with the regular inputs than the background.

It's mostly personal taste. The kind of example and images on linear lens flares are 99% drawn by designers and not based on a physical phenomenon. 

The real deal is achieved by optical filters that are based on dispersion -the grid "printed" on the lense is smaller than the wavelength of the light. 

https://hoyafilterusa.com/cdn/shop/products/hoya-star-six-camera-filter-after-01_720x.jpg?v=1668623886

https://hoyafilterusa.com/products/star-6x

It is similar to the structural coloration of butterfly wings. Would be more challenging to replicate such a look.

https://www.youtube.com/watch?v=Z-bELAv_sOk

Thank you for the explanation, it was very helpful.
I would also like to ask one thing: do you think it would be possible to include this idea/request in some future Inkscape roadmap?

I do not necessarily mean a perfect automatic effect, but maybe:

• better tools for linear flares/light streaks,
• more precise control for “pointy” gradients,
• or some more advanced bloom/glow-type function.

I understand that it can be built manually with masks, gradients, and interpolation, but it still seems like this kind of effect is somewhat inconvenient to create in Inkscape.

Thanks again.

Well it's hard to reply to that. 

Surely everything is inconvenient to draw yet to verbalise how  exactly to help the process you have to experiment and actually come to your conclusion, how you'd achieve the expected result. 

Then, verbalise what you'd expect from a proposed new function. 

Mentioned bloom previously as that can be achieved in blender/other 3D renderers from a light source and metallic material reflections, 

however there you still cannot control the exact shape of the flares /by drawing the exact boundaries, as in a 2D drawing program, simulating a similar effect.

As far as I know Jabiertxof is working on a new kind of gradient, which is somewhat similar to Voronoi -based on separate objects/points.

Also in 2008/2009 diffusion curves was proposed as a new solution to draw custom gradients:

https://artis.inrialpes.fr/Publications/2008/OBWBTS08/diffusion_curves.pdf

But, important to note, inkscape is aiming for covering the svg specs- of which the mesh gradients are already an extra.

To request a feature, you'd have to propose a solution, create some visuals, post it on the github page 

AND get developers interested.

Personally I'd expect it to be done manually within a reasonable timeframe, instead of waiting for years for a new feature to drop. 

(Sidenote, almost finished a drawing with a flare with gradient meshes, then it sudden crashed. Maybe will make another version if you got stuck.)

Weaksauce test... radial gradient.