View Full Version : per patch varyings : arrays vs vectors

10-27-2011, 06:00 AM
When setting a specific component of an array using gl_InvocationID, I get undefined results from patch varyings set in my tessellation control shaders with vectors while arrays work fine. I don't see why vectors should give an undefined result.
Here's some code illustrating the problem (using triangular patches):

// tessellation control shader
#version 410
layout(location = 0) patch out vec3 oData1;
layout(location = 1) patch out float oData2[3];


void main()
oData1[gl_InvocationID] = someScalarValue(gl_InvocationID); // undefined in my tessellation evaluation shader
oData2[gl_InvocationID] = someScalarValue(gl_InvocationID); // works fine in my tessellation evaluation shader
Either I missed something in the specs, either it's a driver bug, I'm on Catalyst 11.9.

Alfonse Reinheart
10-27-2011, 09:57 AM
`oData1` is not an array. So how are you accessing it as an array?

10-27-2011, 10:06 AM
It's perfectly valid to access the components of a vector using the bracket operator (see, for example, page 80 of glsl420 specs: section 5.5).

10-27-2011, 04:48 PM
Tessellation shader invocations were designed to be run in parallel. AMD hardware is vector-based, so I'd guess that what is actually happening in each invocation is:

load vec3 data
insert scalar->vec3
store vec3 data
Because of the fact that the entire vector is being stored, parallel invocations are stomping on the same data location, producing unpredictable results. The float[3] array is very likely being stored in separate 128b-padded addresses, so no data-stomping occurs.

This is in theory, anyway :) -- I don't exactly know what's going on behind the scenes.