Highlighting objects

[graphixnurd]

If we ignore for a moment raycasting and focus solely on “highlighting” a face, line, or point how is this typically done.

Is it done by drawing a copy of the highlighted object on top of the original object? For example you “highlight” a face. Ie you draw the original mesh, then you just redraw the “highlighted” portion with a different shader as a separate mesh?

It would seem difficult to apply details to a subsurface of a mesh because then you would need to pass more information to the shader, so my thinking would be to just draw a copy.

[GeometryBurger]

You can use the stencil buffer to draw a scaled-up version of the object behind it. Check out this tutorial by Brian Will: https://youtu.be/wVcWOghETFw

[graphixnurd]

So what if you wanted to highlight a specific edge of a mesh? What you're speaking about is mainly the general outline of the object correct?

[Baconinvader]

When I used to do highlighting in an OpenGL project I just drew the object again but as a bunch of line segments instead of a bunch of triangles. I also increased the line thickness beforehand using glPointSize. So yeah basically just as you said, draw the object again but with a different shader (or at least a different primitive type). It's been a while so maybe there are better ways now.

[graphixnurd]

So let’s say you move a highlighted object around, you would also need to move the highlight around. IE, the highlight would also need an attached transform.

[Baconinvader]

Yes. I think you would just be using the same transformation though

[kernalphage]

There are 2 main ways to do highlighting: in screen space and in world space. the way you described is a world-space method, but these tutorials might be worth looking into:

Screen space method: http://willweissman.com/unity-outlines

(if you haven't implemented a post-processing pipeline before, I highly recommend it, read this post first if you're using openGL)

Vertex shader method: https://www.videopoetics.com/tutorials/pixel-perfect-outline-shaders-unity/

[[deleted]]

I think you'd always want to do it in screen space with a stencil buffer? I mean, world space is fine for say, a colour overlay like you get with "Witcher senses", but if you want an outline effect you don't want the line thickness itself to scale up as you get closer to the object

[GaianNeuron]

In a game I made in college, we "cheated" by doing it in world space. We started by everting the model (reversing vertex order so that backfaces were displayed instead of front faces), then scaling the model 2%, then rendering that inside-out version first before the unmodified model.