Light through a multi-coloured transparent object

Hi all,

I’m new to this forum. I can’t seem to find an answer to what seems to be a simple question.

I want to shine a light (say a spot) through a transparent object which is coloured - think of a movie projector, where the light shines through the celluloid, and projects an image onto a screen.

Is it possible using raytracing or similar for the colours on the celluloid gel to affect the rays of light that go through the transparency and hit a movie screen?

I’m sorry if this is a basic question, but I can’t seem to find an answer to it.

Thanks for any help you can offer.


You mean that you shine a light through a reasonably transparant piece of paper (multi colour) and some parts block out the light, producing an image on a screen ? Like the ppl who make animals with their hands ? Is that what you mean ?

You could also put a black and white texturemap on a spotlight, and shine it on a screen… that way it’s like you put an image in front of your light…and it’s projected by it …

Not with Blender, but with YafRay


um couldn’t trace-shadow be used for this??

Transpshadow I think only uses the alpha value, not the colours.

Caustics would work, but blender doesn’t do caustics (yafray does, as S68 said).

Best bet would be to have a square spot with a texture map, or to add a texture to the screen with the “projected” image.

yeah it does use color

I don’t know how to make it project images, but it is possible to project colors.
Inside is a file called windows_tra_shadow.blend. Those windows have a texture applied to them; maybe it’s possible too with images?
And I don’t think it’s a very basic question :wink:

you could also just make a picture of the glass window, and have it as the texture of the light

You mean like this, frogmonkey?

You don’t need yafray for this because caustics aren’t involved with projectors due to there being no refraction in the slide. The way I see it, you can do it 3 ways:

method 1

  1. make a mesh for the slide and screen and a spotlight.
  2. map an image onto the slide and set raytransp on. Set fresnel between 1 and 3 and factor between 1 and 1.5.
  3. select the screen (projected mesh) and set it’s material with trashad on so it receives the transparent shadows.

This way is slow because it uses ray shadows. It can cause artifacts on the projection. It’s also difficult to adjust the projected image (especially brightness).

method 2

Just put the image directly onto the screen. This can be done via UVs or just resizing the texture in the map to section. This would mean that if you move the projector, you have to adjust the UVs every time. For that reason alone I don’t recommend it.

method 3

Map the image onto the light. At the right of the spotlight panel, add the image as a texture, make the coordinates use view instead of global. Now, this mapping squares out your image so you have to adjust the size to fit the aspect ratio of your image.

This method tiles your image though (does anyone know how to turn off tiling?), and it also seems to warp the edges (I guess because it’s mapped to a light that projects spherically - so you might have to warp your images to counter this). The good thing about this method is you don’t have to move the images if you want to move the projector.

However, I don’t know if you plan on animating the slides changing. If so, method 3 gets a bit tricky. You could maybe get away with animating the textures but I don’t know how you do this or if you can with lights.

Here are pictures of methods 1 and 3. I reckon method 1 might be best for it being easy and accurate. But you’d have to figure out how to make the projected image brighter. It can possibly be done by adding a texture to the screen and using some effect to darken the colours. BTW, in the first image, the white dots are the artifacts caused by method 1. The second image is a bit squished as I forgot to change the aspect ratio but you can see it’s clearer. I used a halo spotlight so you can see where it is.

Because I’m using geocities, you have to click the link and when you see the page fail, hit return in the location bar. Either that or copy/paste the urls:

Good answer, OSXrules. And OS X does rule. I think every Elysiun user should come read the “Blender General” forum frequently - I always learn something, lol. Keep it up, all.

Just make sure that your transparent object is traceable and shadows are also turned on for it. Then make sure your object recieving shadows has the TraShado button pressed in the material. Also click into the Mirror Transp tab of the material and enable Ray Transp and possibly increase the Depth. Then enable ray tracing and add a light casting ray shadows and render. Should work fine.

in short: you can project a picture easily from a light on a wall, but you won’t see a halo., i.e. volumetrics cast by the image. volumetric shadows work only with geometry.

BTW, a free beer for the first to tell me if and how this (volumetrics through colored image with alpha) can be set up in yafray. i got tired trying different builds, bagisas, yable, trying to post on (this forum definitely needs more life) etc. etc…