Render and preview are different (alpha issue). #41844
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Reference: blender/blender#41844
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System Information
linux 64
Blender Version
Daily build.
Short description of error
Render and preview are different. Possibly this is an issue.
Screenshot (Tested on CPU): http://i.imgur.com/M6ZNc3G.png
File: https://dl.dropboxusercontent.com/u/26887202/blender/test_shader.blend
But! If I put Non-transparent object to backgraund - render and preview will be the same.
Exact steps for others to reproduce the error
Changed status to: 'Open'
Added subscriber: @PaulGeraskin
#104320 was marked as duplicate of this issue
#47388 was marked as duplicate of this issue
We get the same issue with alpha with Fire simulation system on my job. Transparent background works buggy.
Added subscriber: @mont29
Please always attach (valid, camera is missing in yours) compressed (option in Blender when saving) files here ;) test_shader.blend
Sergey, looks like a premultiplied/raw alpha issue?
We also tried to make AlphaOver. But result is a bit different. But almost the same as 3dViewer has.
Sorry, forgot about Camera. Just add default camera.
Render and preview are different.to Render and preview are different (alpha issue).Changed status from 'Open' to: 'Archived'
It's not bug with cycles, it's a specific of visualizing premultiplied image in viewport and image editor. Viewport is like compositing the render result on the checker, image editor converts image to straight alpha first (because ogl is set up in a way it expects straight alpha in blender atm). Hence you've got differences in browsing the same image in viewport and image editor. And what's more funny here -- neither of the ways are correct or wrong -- this totally depends on what exactly you're investigating and expecting.
Not strictly speaking a bug, just a workflow and suecase question. I'll revisit this for 2.73. Thanks for the report anyway.
Ok.
Just don't forget to add it to 2.73. We need some solution for alpha.
Right now we cannot use any Additive Materials in Cycles. And this is really needed feature, especially for gamedev.
Alpha is not seen for additive things :( http://i.imgur.com/AIe0lsK.png
Also, how you will solve it in 2.73?
Thanks.
Well, the thing is -- the underneath data is correct, it's just image editor which displays it differently from the viewport. To answer your question about fixes -- what exactly usage of the render results? As in, how they're being handled after the render engine in your pipeline?
With Additive Materials we achieve visual effects for waterfalls, engines, particles, lighting effects, etc. Additive materials are used in many engines. Even game engines like Unity/Unreal has "additive" checkbox.
Example of engine with additive material: http://wiki.blender.org/uploads/0/0a/Additiveaproach.png
All we need is just to get the same result as viewport has. Without premultiplication. Or have possibility to switch premultiplication off.
Here is another example i showed you https://dl.dropboxusercontent.com/u/26887202/blender/fc_buildings_198_frankenstein_laboratory_01.zip
Just render the blend file and there will be no alphe for additive material.
In the buildings file it is a proper additive material, which represents no opaque and just light emission, meaning alpha=0 and rgb is set ot the emitted light. This is the only way you can composite layers together, and that's what's gonna to be stored in exr file.
Uncertainty begins when you want to make this light visible but keep it still semi-transparent. There's no easy answer on this how to achieve. And no proper math, for as afar as i concerned. One of the tricks would be to set alpha to the max or R,G,B,A of the input image and convert alpha from straight to premul in compositor. That image would be displayed and saved to formats like png in a way you expect it. This might give some major headache when compositing the final shot tho. But as said, for compositing it's all correct without tricks.
So yeah, it's rather a tricky topic, need more research in there and it might still be no "one-click" solution for some usecases.. Anyway, will try to make things at least more clear for the next release.
Thank you!
I hope it will be possible to achieve for next release.
You are our super-hero! :)
Added subscriber: @Sergey
@Sergey hello again. We noticed that alpha is changed in fire simulation too.
Just see the difference betwee peview and the render http://i.imgur.com/1MU8Q1O.png
I attach the file. Try to render 20 frame. smoke_particles_effect_01_005.blend
Changed status from 'Archived' to: 'Open'
@Sergey Hi.
2.73 was released, but we still have big issues with alpha. Semi-transparent materials become ugly on final rendering. Could you review this bug again? You said that you would fix it in 2.73 but it did not happen. Could you make a simple checkbox for turning alpha-premultiplication off for final render?
Here we still suffer from rendering semi-transparent materials. See Green glass:
Here is the blend file again if it helps you:
temp_alpha.blend
I'll Open the task for a while if you don't mind.
Thanks.
Didn't say i will fix it, i told i'll review the status of alpha in the image editor. It's just two different approach to visualizing the images, just making it so image editor starts behaving same as viewport is not totally correct and might ruin someone's day,
This is still on the list to be sovled, but since it's how blender always worked its closer to low-prio todo.
Added subscriber: @MassimilianoPuliero
Added subscriber: @Vyach
Probably the same or similar issue.
Final cobined result is not combined with separate glossines. I have a scene with monocle. So I need normal material for metal and only reflective for glass.
here on the screenshot, in the bottom row are:
preview,
combined result (empty)
Direct reflection
Reflection color
And if I will have separate render-layer with such material, and if I will disable sky visualization, my lense starts reflect like sky is black.
So if the sky are absent it is nothing to reflect! Not white, nor black!
In is not physically but it is useful for making pictures.
Added subscriber: @KonstantinsVisnevskis
Just run into this. So basically - viewport preview doesn't work for semi transparent objects as what you see is not what you get. I understand that probably output can be changed into what is shown in viewport theoretically, but I wasn't able to do that in compositor. I can get a same image as if viewport would be rendered over some color, but not if keeping the alpha (adding un/premultiply seems to fix alpha issue, but brightens the colors, that doesn't mach the viewport again).
Added subscriber: @PranavjitVirdi
i am having a similar issue, 2.79 official release, and had it earlier aswell
Added subscriber: @brecht
Changed status from 'Open' to: 'Archived'
Duplicate of #52680 (Alpha difference Viewport/F12 ).
Added subscriber: @JasonForth
This is still an issue in 3.2.2.
I am trying to render an animation, but the resulting images being produced are nothing like final renders within Blender.
I have tried to change the colour management options, but all yield the same result. I removed the option of using alpha and the render came out how I wanted it to, however, having a solid background on this animation is not an option for me.
Is there a workaround at all?
Added subscriber: @Vasiliy
Issue with alpha still here 3.4.1
Render view on the left, on the right how it actually look.
