"Images as Planes" (texture with transparency) + Cycles, plane still visible after render

Question

I have quite a dense scene, with a lighting from behind through fog. I like the mood very much. Here is an EEVEE render (16 samples):

When I render this with cycles (512 samples) it looks like this:

One can clearly see the planes, where the images of the textures are placed on.

Before the scene was that dense, I made a cycles render with the same settings and got this:

I'm keen about knowing more about the effect in cycles. I believe, because the more dense scene leaves less less light will reach the planes in front of the camera and this will cause the effect. But I wonder why the transparent plane parts are affected by the rendering at all...? Because they are... transparent!?! I wonder if insanely high sample values in cycles would improve the situation or if I just have to have a more powerful light. In the end this should be an animation of at least 300 frames.

(like here in EEVEE: https://youtu.be/f4u1mUMdepI)

Edit: This is my shader node tree for the "Image as plane" object of one of my flowers:

Edit: This is the image on a plane that I use as texture. I cannot see any non transparency on that plane though...

Edit: Well, maybe there is something, because the grid does not shine through as bright as it is besides the plane. Looks as if I made some fundamental mistake by creating that image for my texture...

Answer 1

This problem may have less to do with your shader-tree, and more to do with your render settings. Blender's Cycles renderer is built for realism, but by nature, Cycles is still a little bit of a biased engine, to avoid obscene render times on simple scenes.

Let's tell Cycles to put a little more effort where it's needed:

Go to the "Render Properties" tab, (looks like a small camera icon) and click on "Light Paths".

Under Light Paths, locate the section that reads "Transparent".

Change your "Transparent" Light bounces from "8" to a higher number, like "32" or "64".

(I recommend finding the lowest value that works for your scene, and rounding it up high.)

Good luck! Happy Blending!

Answer 2

I'm sorry about the delay, but here's the answer I promised.
This image shows how I usually use transparency from images:

I did a quick test with the Alpha socket of the Principled BSDF, and it seems to give the exact same result, though the setup in the image also works with shaders that don't have an Alpha socket. I doubt if the setup described here will actually make a difference, but one point of difference (that may or may not matter at all), is that I always change the interpolation of image textures to Cubic. I also explicitly include the UV, but that really shouldn't matter, as for image textures the default UV map is used implicitly. My best guess is that the alpha channel of your texture isn't fully transparent. You could, as @RonJensen suggested, insert a Color Ramp to make sure the alpha value actually is 0 where it should be.

The Mix Shader mixes two shaders such that if the Fac is 0, it's equivalent to using only the shader that is connected to the upper Shader socket, and if the Fac is 1, it's equivalent to using only the shader that is connected to the lower Shader socket, and anything between 0 and 1 mixes the two shaders correspondingly.

Why your problem doesn't occur in Eevee, I honestly cannot say for certain, but I seem to vaguely remember having been told that Eevee only has on or off for transparency, no in-betweens, unless explicitly enabled. This would make sense from a performance point of view, but accuracy would of course suffer.

Answer 3

I'm extensively embarrassed...

As Pierre suggested, I wanted to upload my blender scene. It was way too big and so I started to reduce the objects in it.

As mentioned initially, the effect occurred when the density of the scene increased. So I thought, at one point, the effect will disappear, while I reduce objects... In the end I had less objects as in my initial question, but the effect was still there.

Then I saw it... I somehow, accidentally doubled the plane on which all the particle textures were placed... By doing this, I inflicted the effect by myself because now, planes were just on top of each other and I cannot blame cycles for running wild here. I'm sorry that I wasted everybodys time!

To make it clear, here are some before/after screenshots:

However... as shown in blue, there are still artifacts of the plane visible. They disappear more and more, when I increase the overall render samples. The lightpath setting that Pierre suggested does not help here, either. I still wonder why a transparent plane has such effect in cycles...

Anyway, I still tried to upload the file for you to examine (the "blue circle" effect), but I cannot reduce the file size under 49MB and I'm only allowed to upload 30MB. Since the main question is answered, I think this question can be considered as closed. Thanks everybody! I learned a lot!