glsl highp mediump varying inconsistency ?

[Evgeny Demidov]

at
https://webglfundamentals.org/webgl/lessons/webgl-shaders-and-glsl.html#varyings
(and in all my shaders :)
In the vertex shader varyings have default highp precision (e.g. varying
vec4 v_positionWithOffset) but in the fragment shader they are mediump
floats
precision mediump float;
varying vec4 v_positionWithOffset;
How does it works? Shall (can) I declare in the vertex shader
varying mediump vec4 v_positionWithOffset;
and (just curious) can I sum in the vertex shader
v_positionWithOffset = a_position + u_offset;
as mediump floats?

Evgeny Demidov

[Cătălin George Feștilă]

I don't know this website , but is very good to start it.

About precision mediump float declarations and functions used by shaders  I know has some changes ito specs 

You can find some ideas here: 

https://shadertoyunofficial.wordpress.com/2016/07/22/compatibility-issues-in-shadertoy-webglsl/

Evgeny Demidov

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[Kirill Dmitrenko]

Doesn't seem so according to specs. GLSL 1.0 spec defines link error if varying **types** mismatch (11.4). However, 4.5.2 says "Precision qualifiers, as with other qualifiers, do not effect the basic type of the variable." Thus, a program with varyings with different precision qualifiers, but same types, should link successfully. More over, same precisions qualifiers on different pipeline stages may provide different precision (threre's nothing in the spec preventing implementations from doing that). Also, varying aren't passed from vertex stage directly to fragment stage but rather go through fixed-function hardware that most probably doesn't recognise precision qualifiers at all.

четверг, 25 мая 2017 г., 9:47:12 UTC+3 пользователь Evgeny написал:

[Kenneth Russell]

The precisions of varyings can differ between the vertex and fragment shader. See the section "Precision Qualifiers" in the OpenGL ES Shading Language 1.00 specification at https://www.khronos.org/registry/OpenGL/index_es.php .

If your vertex shader is running with good performance using the default highp precision, I would recommend not changing it.

On the other hand, to work on the broadest range of mobile GPUs, it's best to use mediump precision in the fragment shader. highp support is mandated as of OpenGL ES 3.0 (WebGL 2.0), but still, using reduced precision will probably perform better on a broader range of GPUs.

The precisions of uniform variables shared across the vertex and fragment shader must match. This is defined in the ES 2.0 spec.

-Ken

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