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There is an option in the properties panel, Render > Light Paths > Shadows that toggles "using transparency of surfaces for rendering shadows" for the whole scene.

I like to render architectural stills or short films. When I uncheck this option the rendering speed greatly increases, but some parts such as fences and curtains don't work very well, although the quality of shadows of trees, grass and so on is sufficient.

How is it possible to change this option per-object?

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There will be a "Transparent Shadows" option per material in version 2.68 and the latest Blender builds.

Using a Light Path node to disable transparency helps and achieves the same visual result, but it still needs to execute the shader to find out, which costs some performance.

Update:

This is now accessible in materials > settings:

enter image description here

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  • $\begingroup$ I thought that when a mix shaders factor is exactly eqal to 1 or 0, then the other shader is not calculated. Cycles could do such optimizations. Anyway, is the mentioned option already in trunk? $\endgroup$ – Róbert László Páli Jun 18 '13 at 15:45
  • $\begingroup$ That's not the case unfortunately, if there is another node plugged it doesn't always skip things (unless you use Open Shading Language which has smarter optimizations). This Transparent Shadows option is in trunk now. $\endgroup$ – brecht Jun 19 '13 at 21:56
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I think you could edit the node tree on the materials that are transparent, but shadow should not use transparency, the following way: Before the output node, add a mix shader that mixes the original main shader with a simple solid black shader, and plug the IsShadowRay input to the mix factor. This should theoretically do it. Currently I am at work, so no Blender here, later I might try it and add images.

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