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Why is the same frame never the same?

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I'm currently going through Marijn Raeven's tutorial on Natural Environments from HelloLuxx. It is only for still images, not animation.


However, I can't understand why, when I render exactly the same frame, immediately after it has rendered once, that it's different?


In the scene there's a stream so has a texture with a simple noise bump and then some roughness, specular (Beckmann) and fresnel in the reflectance channel. It also uses the Physical Sky. On rendering, I just render out a section of the stream and the specular highlights are never in the same place - when A/Bing the two images in the Picture Viewer, they jump, even if slightly.


This is with and without GI... with both the Physical and Standard renderer, even on very high settings.


If someone gives me an algorithm of, say x = a + b / 2 where a=5 and b=3, then the answer is always x=4. Never x=3.8 or 4.2 or whatever - so why, when a rendered calculates, it come up with small differences each time?


Are the built in renderers not up to this kind of job? I was reading through a greyscalegorilla article and someone commented "You should add one more con for Physical, Flickers in animation!!!!" Or do some settings have to be cranked way up so each frame will take an hour or more?


Thank you for your help



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On 1/30/2019 at 3:16 PM, Renderer said:

In the scene there's a stream.... I  just render out a section of the stream,,,,

I think there is some physics/dynamics involved? You tried baking the dynamics? You  need to bake it to get it exactly precise every frame.


Noises should be exact, so that is not the problem.


But yeah, post a simplified example.

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  • Sorry for the delay... been doing lots of tests and can't get things to work unless I go to very high render settings which just take too long to render.


    Unfortunately, it is noise - there are no dynamics or anything. Just a very very basic scene - I've attached a very basic example - just some terrain, a stream and cloned cubes (representing trees!) and a physical sky.


    I've reset the render settings back to zero so, yes there's noise, but surely, even at this level, the noise should be in the same place if it uses the same algorithm???


    I had to zip the file as it was about 11MB.


    So, just open the file in C4D, render to the Picture Viewer and as soon as it's rendered, render again immediately and then switch between the two images - the changes should be evident, but if not, look at the trees (at the lower settings) but the far side of the stream changes even when the "trees" look ok.


    Thank you for any help!


    Edited by Renderer
    Just adding instructions.

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    You are using too little samples and in adaptive. Once you hit render, rays are randomly sent to scene and because their number is low they will hit different areas then on each new render. That is just a nature of rendering. Better use fixed samples and higher quality to send enough rays and you will eliminate this. Low sample and quality is just for previews...

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  • Thanks HSrdelic... Yes, I know that I have to crank up the number, and I did render it with Fixed and had to get to a value of 6.5 subdivisions before I could get exactly the same image! That meant at very large render time (on my original scene) which was unacceptable.


    I have since tried the Redshift demo, converting the materials and using the RS Sky... and, on rendering, the (very low level of) noise is in exactly the same place every time - so the algorithm isn't random like Physical - and it rendered a lot quicker too.


    Even the ProRender renderer had it's noise in exactly the same place... but ProRender is no good at the moment - it doesn't even fully use the Physical Sky yet.


    So, I think C4D with its own renderers is only much good for photo realistic stills, or animations with basic shading. Unless you want to wait hours per frame! C4D is like having a word processor where you have to buy a plugin to print a document to be exactly the same for multiple copies... Or wait hours per page. Not great!


    I'm going to try to recreate the tutorial in Blender 2.8 and see how Eevee goes.

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  • Been doing the first Redshift tutorial on the Redshift site as they have a discount this week...


    The main reason I like Redshift is that when I render the same frame you don't get a different "noise" pattern - also no "fireflies" in animation.


    And the times are much faster - even just on my laptop here with a GTX970M nVidia card (all images were 1920x1080)





    (this was on Progressive mode - 32 passes)



    Standard is faster, but noisier - and also the same noise pattern / firefly problem.






    ...and just some other quick tests with Redshift only...






    With their $100 off this week, I'm very tempted, and with the "real time" preview as you're changing materials, camera position etc, it's pretty good too! Still, $400 for something that should be part of C4D already :sad: just adds to the cost of this software. Yes, I could crank up the settings and wait for around an hour a frame to minimise fireflies, but that just isn't worth waiting for when you know it doesn't have to take so long!

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