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enter image description here

I found this example in this Chinese language document.

In the image above, I can understand the value of SV_TessFactor because for every side of this triangle, there are 4,1,6 line segments respectively. But I don't understand how InsideTessFactor is 5.

There are 12 points and 12 line segments inside the triangle. It seems to me that 12 is not relevant to 5.

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You can understand this as a continuation of the two preceding diagrams in the document you linked:

Tessellation Sequence

For an inside tesselation factor n, we first imagine that each of the sides of the outside triangle have been subdivided into n segments.

Then we draw a perpendicular up from each of the newly-added vertices. Where the two perpendiculars closest to a corner intersect, we place the corners of our next inner triangle (blue).

The other perpendiculars add vertices where they cross this triangle's sides. That means that for a triangle tessellated by a factor of n, this inner triangle has n - 2 segments along each side.

Then we repeat to make a nested inner triangle (or point), until we have no more vertices in the middle of a subdivided edge to draw perpendiculars from.

So when the inside tessellation factor is 5, that first inner triangle in blue has 5 - 2 = 3 segments along each side.

You'll note this inner 3-segment triangle for the 5 case in my diagram matches the 3-segment inner triangle in your question. All that differs are the number of segments on the outer triangle's sides, controlled by the other tessellation factors.

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  • \$\begingroup\$ Thank you so much,DMGregory,this perfectly solved my question.The illustrations here are very informative. \$\endgroup\$
    – Mark Peter
    Feb 26, 2021 at 2:36

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