View Full Version : Colour filter and Caustics

05-29-2003, 04:10 AM
I'am using Colour Filter under the advanced tab in surface attributes, to give me a nice colour filtered shadow with my transparent objects, but when I turn on caustics it totally ruins the colour filter affect and my shadows return to black.

Is this a 7.5c bug?

Using caustic together with colour filter would otherwise be a great option.

If you render the scene first with caustics off, and then on, you will see the problem.

here's a test scene.

05-29-2003, 03:15 PM
I can't really see your problem...there simply were no caustics visible at all, so i cranked them up to 1000% intensity and got this:


I think Newtek should really rework the caustics engine, it is extreme dependant on the overall scene size...in small scenes (distances a fraction of 1m size) waaaay to bright and in large ones you don't see them at all...at least that's what i experienced.

05-30-2003, 02:28 AM
It's basicly a two light setup. clone your light and leave it in the same place. make sure that your first light (the one that cast black shadows) doesn't affect caustics. make sure your second light doesn't affect diffuse and the that the ground plain is excluded. This should give the effect that you want. the advantage is that any object using the color filter will cast it's own color shodow. this should prevent you from having to setup a color light for each object.


for good caustics go into the 1000s and higher.

05-30-2003, 02:51 AM
Thanks very miuch for the replies but I think you've missed my point, making the caustics look nice isn't the problem, it's that when you do the render with 'colour filter' on, and caustics turned off, you get the nice colour filtered shadow, but as soon as you enable caustics - sure you can get a nice caustic effect, but your 'colour filtered' shadow completely dissapears, returning your shadow back to black. what I need is both caustics on + the colour filtered shadows. You don't appear to be able to have both.

I'll post an example later.


OSX, 7.5c.

05-30-2003, 03:38 AM
Yes wizlon.

I've tried yesterday spending all the day banging my head against the screen! :rolleyes:
You're completely right: caustics kill color-filter for shadows; I've tried everything possible.

I've stareted a new Thread in "LW - Feature Requests" forum:
Caustics kill Color-Filter in shadows
but no response at now.
Come on there and help me to explain this bad LW's render-engine fault!


05-30-2003, 03:44 AM
Thanks exper, it's nice to know someone else is having this problem. Not that I wish you to have problems - you now what I mean.

Yes It's a real problem, two things that really should work together, don't.

Thanks for starting the other thread.

I'll post my examples there later.

05-30-2003, 04:38 AM
hcoat... you're trick works quite good.

But caustics and color-filter should work together without the need of a trick! :(


05-30-2003, 05:28 AM
Thanks hcoat, that's a good workaround.

05-30-2003, 06:33 AM
Wait a minute, i think you guys are missing the physics behind refraction...

If you don't use caustics you get a blue shadow there, yea...but actually, that's not what happens in reality i think. Since all the light gets refracted when it hits the glass surface there is more or less no light that travels straight through the objeect, casting a blue shadow!
The shadow HAS TO BE BLACK, the only light falling through are the caustics...

So what hcoat did is actually unrealistic...but if you want it like that, no one stops you :)

05-30-2003, 07:39 AM
unfortunatly I've in real world observed coloured things like these expecially in conjunction with caustic phenomenons; color-filter is there for this purpose.
And if it works in RayTrace then it should work also with "Caustics ON".


05-30-2003, 10:41 PM
remember when you used to use a magnifiying glass to focus the sun's light to fry some unsuspecting ants? As I recall, the magnifying glass cast an Opaque shadow, except for the hot spot in the middle.

Lynx3d is correct. If light passing thru an object is refracted, how will it color the shadow, outside of the "caustic" area? *But* maybe not every light ray is refracted. Hence 2 lights makes sense -

If anything is incorrect here it's the accuracy of the Caustics, which is too intensive for our comptuers to calculate in a reasonable time, or the shadows, which are faked. Shadow mapped and Ray-traced shadows are not an "absense" of light, but are cast onto objects under it (as near as I can tell )

Maybe Radiosity combined with Caustics is what we need ( and a Dual P4 3.06ghz! )

05-31-2003, 05:09 AM
Hehe, yea toby, i agree.
One thing i've notices, it seems as if area lights are treated like point lights when calculating the caustics, if you put several point lights in a square shape you'll get much wider spread caustics that look more believable! I think i'll experiment with the good old spinning light trick...

And in reality there's always some ambient light that will fall through the object and possibly tint the "hard" shadow area, but to simulate that not even radiosity does it completely i think...

06-01-2003, 04:59 AM
Check out these tests i did a while back...

There are NO lights and caustics are OFF in these scenes, just luminous objects and radiosity!