fred_em

05-03-2011, 09:43 AM

Hi,

How do I write my control shader so that I disable the inner tessellation level altogether ? Please, please tell me there is a way to do this.

I want to tessellate my patch so that I end up with the following tessellation:

x----x----x

| /| /|

| / | / |

| / | / |

|/ |/ |

x----x----x

I tried with

gl_TessLevelOuter[0] = gl_TessLevelOuter[2] = 1; // left and right edges

gl_TessLevelOuter[1] = gl_TessLevelOuter[3] = 2; // upper and lower edges

No matter what I do with gl_TessLevelInner[0] and [1], I can't get it right.

Any idea?

Thanks,

Fred

PS: sorry for the multi-post. I actually posted this question to the beginners forum, but realized a short time later it was more appropriate to ask the question here.

fred_em

05-03-2011, 10:34 AM

I managed to get something that LOOKS alright by doing this:

gl_TessLevelOuter[0] = gl_TessLevelOuter[2] = 1.0; // left and right edges

gl_TessLevelOuter[1] = gl_TessLevelOuter[3] = 3.0; // upper and lower edges

gl_TessLevelInner[1] = 1.0; // left / right

gl_TessLevelInner[0] = 3.0; // upper / lower

Result:

x----x----x----x

| /| /| /|

| / | / | / |

| / | / | / |

|/ |/ |/ |

x----x----x----x

In the tessellation evaluation shader, I must have fractional_odd_spacing:

// Note: fractional_odd_spacing

layout (quads, fractional_odd_spacing, ccw) in;

In this situation, things work fine. I need to have an odd number of subdivisions (1, 3, 5 and so on.).

Not sure if this whole thing is correct since the documentation is a bit difficult to understand (ouch)

Am I sure exactly 8 vertices and 6 triangles are generated with the example above (tess levels = 1.0 / 3.0) ?

If "equal_spacing" is used, the floating-point tessellation level is first

clamped to the range [1,<max>], where <max> is implementation-dependent

maximum tessellation level (MAX_TESS_GEN_LEVEL). The result is rounded up

to the nearest integer <n>, and the corresponding edge is divided into <n>

segments of equal length in (u,v) space.

If "fractional_even_spacing" is used, the tessellation level is first

clamped to the range [2,<max>] and then rounded up to the nearest even

integer <n>. If "fractional_odd_spacing" is used, the tessellation level

is clamped to the range [1,<max>-1] and then rounded up to the nearest odd

integer <n>. If <n> is one, the edge will not be subdivided. Otherwise,

the corresponding edge will be divided into <n>-2 segments of equal

length, and two additional segments of equal length that are typically

shorter than the other segments. The length of the two additional

segments relative to the others will decrease monotonically with the value

of <n>-<f>, where <f> is the clamped floating-point tessellation level.

When <n>-<f> is zero, the additional segments will have equal length to

the other segments. As <n>-<f> approaches 2.0, the relative length of the

additional segments approaches zero. The two additional segments should

be placed symmetrically on opposite sides of the subdivided edge. The

relative location of these two segments is undefined, but must be

identical for any pair of subdivided edges with identical values of <f>.

Source: http://www.opengl.org/registry/specs/ARB/tessellation_shader.txt

Fred

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