The bounding box is the area c2 uses for the effect. It does that as an optimization. The reason the outline of the box could be it’s sampling pixels outside that, which this effect does. Maybe it’s undefined behavior or maybe it’s something that can be handled. If it is it would be in the c2 renderer and not in shader.

The reason it draws darker or only happens when transparent could be multiple reasons. One, perhaps doing an outline only works when no transparent and the opacity should be applied later? This is something that maybe can be addressed at the runtime level? I could be wrong there since I have minimal understanding how effects and the c2 renderer work together.

You could perhaps work around the issues in this shader by utilizing the paster plugin to get more control of the rendering pipeline. This may not be feasible if the effect doesn’t draw right, since there are issues drawing effects with it. I haven’t tested it, but I digress.

Basically you make the objects not transparent and paste them to a paster object with the outline effect. Then you make the paster object semitransparent and paste that to another main paster... it sounds more complicated than it is.

The object is to have everything invisible and instead just draw to the paster.

So the solution is to either:

1. Fix the shader. No idea what a fix would be.

2. Use the paster object to do the rendering in a more manual controlled way. It may be tedious.

3. Modify the renderer? Least feasible of all. No idea what you’d change here if anything. Probably doing some investigations with straight webgl and shaders in straight JavaScript would be enlightening. Maybe a solution would be clearer after that.

Anyways cheers

Edit:

It occurred to me that you could try increasing the bounding box size (there’s a setting in the shader xml), then in the shader itself skip when processing pixels around the edge. It should be doable I think by dealing with the uv cords. I feel like I have to re-figure out how they work in shaders every time I use them so I’ll pass.

How to make a stroke transparent? could you write to formulate this task?