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Clarify behavior of POINTS draws when gl_PointSize is unassigned. #3370

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Clarify behavior of POINTS draws when gl_PointSize is unassigned. #3370

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22 changes: 22 additions & 0 deletions specs/latest/1.0/index.html
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Expand Up @@ -4455,6 +4455,28 @@ <h3>Wide point primitive clipping</h3>
</p>
</div>

<h3>gl_PointSize must be set in vertex shader when drawing POINTS</h3>

<p>
If <code>POINTS</code> primitives are drawn, then the vertex shader for the currently bound program
must set the built-in output variable <code>gl_PointSize</code>, or the draw will have no effect.
</p>

<div class="note rationale">
<p>
The OpenGL ES and OpenGL GLSL ES specifications define that if the built-in vertex shader output
variable <code>gl_PointSize</code> is not written to, then its value is undefined in later shader
stages. As a consequence, in these specifications, it is undefined whether the draw has an effect.
</p>

<p>

The WebGL API clarifies this behavior by guaranteeing that such draws have no effect. All of the
validation for the draw operation, including generation of any errors, is still performed.
</p>
</div>


<!-- ======================================================================================================= -->

<h2>References</h2>
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