i have a question about the lighting in cycles.
If i use an hdri background, is it necessary to use also a sun lamp?
If your HDRI properly encodes the sun. no. However, many HDRIs have the sun clipped and thus WAYY dimmer than it’s supposed to be. You’ll need to fudge the extra light back with the sunlamp in this case. There really isn’t a precise way to do this, unfortunately. You’re just fudging back data that should’ve been there in the first place.
I’ll use hdri only (where I scale up the brightest intensities) for testing only, like during modelling and material testing. For proper scenes I’ll always use a sunlamp as it cleans up much faster than hdri’s. It’s currently a pain to setup. At least upcoming is the ability to hook up light direction with sky texture, but I’d also like to just click in view with hdri as background and by maths (and magic?) the sun is oriented correctly.
When you are considering this sort of questions about Cycles, I strongly suggest that you contemplate the following discussion of the algorithm that is represented by this particular rendering engine:
From here:Cycles is a “backwards” path tracer, which means that it traces light rays by sending them from the camera instead of sending them from light source(s).
Then, from the cited WikiPedia article:Path tracing is a computer graphics method of rendering images of three dimensional scenes such that the global illumination is faithful to reality. Fundamentally, the algorithm is integrating over all the illuminance arriving to a single point on the surface of an object. This illuminance is then reduced by a surface reflectance function to determine how much of it will go towards the viewpoint camera. This integration procedure is repeated for every pixel in the output image. […]
Path tracing naturally simulates many effects that have to be specifically added to other methods (conventional ray tracing or scanline rendering), such as soft shadows, depth of field, motion blur, caustics, ambient occlusion, and indirect lighting. […]
[…] In order to get high quality images from path tracing, a large number of rays must be traced to avoid visible noisy artifacts.
It may well be said that “Cycles derives its power by looking at the entire rendering problem back-asswards.”
(And-d-d(!!) by leveraging the parallel-computing power of a GPU chip … which is absolutely unlike how a CPU chip would approach the same problem … to enable it to do so.)
Thus: “which one is the cart, and which one is the horse?”
- Ray-tracing (BI) starts at your sun-lamp.
- Path-tracing (Cycles) ends with it (and, simultaneously, “every other possible source of light”).