Black patches in glass when using cycles render engine

Question

For some reason I am getting these black patches all around my image. It seems to be most obvious where certain sections overlap slightly, but I have no idea how to fix them (very new to Blender o.o)

I'm not sure why this is happening. Here's a version where the points I'm referencing are circled:

I don't know what settings you guys need to know so just ask me if you need to. I can tell you this was only rendered 10x but even at 20,000 samples it did not fix anything.

Keep in mind, I am still a noob at this so any suggestions please word as simplistically as possible xD thank you!

Answer 1

This is caused by sharp, low poly geometry with smooth shading (you'll notice that the problem goes away with flat shading). It's true that bounces can also cause dark spots (as mentioned in the other answers), but I think this is the issue in your case.

There a couple things you can try:

• Adding more geometry (works well for curved objects)

  No subsurf:
  Subsurf level 2:
  

• Using an edge split modifier (works better on objects with sharp edges)

  Without edge split:
  With edge split:
  

Answer 2

The black spots are due to the depth limits in the transmission rays. You need to increase the Transmissions rays so that light can gr thorough more objects and the Glossy Rays so that you do not get black reflection from other object (letters) in the scene.

Default Render Settings:

Increased Depth:

NOTE: that the total number of bounces for each ray is set by the Max parameter under bounces. You also need to increase this if you really want to go over the default.

Answer 3

Vaders answer is correct; the number of times a specific type of light ray is permitted to bounce before it is terminated is the problem, and is also correct that it is the Glossy and Transmission ray bounces need to be increased.

The reason you saw no improvement when you made the suggested adjustments is that those values are set in two places.

The value you set for each bounce type (outlined in blue) is the limit for the maximum number of times that type of ray can bounce before it is no longer calculated, depending upon your scene however, it may be terminated before reaching that limit. But; those limits are constrained by the Min and Max values (outlined in red). By way of example; in the image below, despite having a setting of 1, 3 Volume bounces will be calculated, and entering a value of 50 for the Transmission bounces, still only 12 bounces would calculated as the Max value will terminate the ray at 12.