# Why is the decimate ratio being ignored when using Vertex Groups?

I have a 30 million polygon object I’m selectively attempting to make lighter in some areas, however the decimate ratio is definitely being ignored when using Vertex Groups:

That is definitely not a 0.8 ratio, it’s more like 0.001.

Anyone know why Blender behaves like this?

As a help says: “A vertex group that controls what parts of the mesh are decimated.”
So its not for controlling of ratio but just for ON/OFF, unfortunately.

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Thanks, but oof… that goes on the list!

Your last message on those thread are actually not true. Ratio are still in touch if you have some vertex group enabled in the modifier, but it start to behave really wierd and a bit unpredictable.
You still can have even multiple vertex groups and change decimate ratio for different selected areas, but probably would be better to find some other options for any kind of “selective/painted decimation”.

I’m confused by your answer. You seem to say that it does and doesn’t work?

Also, since I tested it twice now (once for this thread and once for the post in the other) and couldn’t get it to work, would it be possible to explain in a third way? What am I missing?

Ops, sorry. Yup it does work, but in a bit wierd way, probably, not the way you can expect it to work.
It cant work like a gradient with smooth changing of decimation ratio. Even if you have a gradient, it will work in a “stepped” way.
Here’s a plain with one vert group with a gradient:

Here’s what we will get with same single vertex group which divide the plane into two area, left area are 1 and right are 0:

So the vertex group will told the modifier to decimate or not in a binary way, but you still able to use ratio slider to set how much it should decimate.

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Ok, so since you say it works and I still don’t get how I tested some more…

Turns out, any value below 0.8 just decimates everything using a vertex group:

Removing the vertex group and changing nothing else produces this:

In order to get a similar tesselation as with a vertex group, you have to put the value at 0.02:

So the ratio works… but it doesn’t feel like the same ratio as when you’re not using a vertex group to mask! Apparently, the ratio slider doesn’t update to max out at the vertex group total, and instead it still uses the mesh total.

In other words, “technically correct”, but the UX suuuuuuucks.

Thank you for testing this for me!