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I know that Blender Internal is a scanline renderer, but how exactly does it work? When compared to Cycles, it's much less accurate, but much faster; Also, it doesn't have the concept of sampling, so, it finishes, as opposed to Cycles, which could theoretically keep going for ever.

How does Blinder Internal work?

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The main difference is Internal is not physically correct, Cycles is physically correct (if you want you can do unrealistic - toon - in it also). This means Internal 'fakes' stuff, Cycles simulates it.

  • Internal uses raytracing and raytraces reflections and refractions. If you want them blurry it fakes it with blurring samples. Global Illumination is faked using radiosity which takes also samples. Cycles uses Path-tracing that can do GI correctly.

  • Specular reflection in Internal is separate thing from reflections, its a fake effect controlled with a Gauss ramp and you can set it completely unrealistic: you can have soft specular and non-blurry reflections or specular of crazy colors. Cycles materials are all reflective, that means they change color based on surroundings. They will look correctly whether you light them with blue light or warm light. There is no such thing as specular reflections, only reflection and roughness. There are different shaders simulating different surface properties that can be combined together. enter image description here

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  • $\begingroup$ That's a little bit besides the point... I already use Cycles, no need to sell me on that. :D $\endgroup$
    – someonewithpc
    Commented Dec 11, 2014 at 19:09
  • $\begingroup$ Sure np, I wrote the answer in a general broad manner.. $\endgroup$
    – Jaroslav Jerryno Novotny
    Commented Dec 11, 2014 at 19:11
  • $\begingroup$ Cycles also fakes stuff, sub surface scattering is fake probably using dipoles, doenst do spectral calculations also I think.. $\endgroup$
    – Jaroslav Jerryno Novotny
    Commented Dec 11, 2014 at 19:13
  • $\begingroup$ Yeah, but I would hate to see the render times if it didn't! I wish there were a few enable-able options for each of those, checked for photorealistic un-checked for speed. It would be easier to optimize renders and to balance between realism and render speed . . . $\endgroup$
    – J Sargent
    Commented Dec 11, 2014 at 21:53
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    $\begingroup$ This is not necessarily correct imo, It's a bit of an overstatement to say Cycles is "physically correct". It simulates light in a physically based way, but that does not mean that it's accurate or "correct". $\endgroup$
    – gandalf3
    Commented Dec 12, 2014 at 6:49
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The difference between BI and Cycles is the difference between a biased and unbiased renderer.

BIASED RENDERING

A biased renderer uses shorcuts to achieve the final image (fewer samples on every pixel, averaged out by the engine, and certain effects are pre-calculated in dedicated passes). It does not simulate how light behaves (at least not fully). Blender Internal does not have indirect capability, so that helps in speed too.

The advantages of a biased renderer is speed.

UNBIASED RENDERING

Unbiased rendering similates how light behaves. While Cycles can be tweaked to defy physics (certain brightness/contrast node combinations that exceed the brightness of what its reflection should be), it simulates the photons (though in reverse direction, tracing the rays from the camera to the source). Too few photons (samples) will produce a grainy image, as under-exposing a camera sensor or film would. There is a lot more math involved, and as a result, is slower than biased ray tracing and even slower than rasterization.

The advantage of an unbiased renderer is physical accuracy.

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