topic OpenGL mix() function not working correctly with Minecraft shaders. in Developing Games on Intel Graphics

OpenGL mix() function not working correctly with Minecraft shaders.

djmrFunnyMan — Tue, 11 Feb 2025 16:56:37 GMT

This is a continuation from https://community.intel.com/t5/Developing-Games-on-Intel/Driver-level-bugs-causing-issues-in-Minecraft-amp-OpenGL-shaders/td-p/1664368 which will be split into 3 separate reports.

The issue:

The OpenGL mix() function seems to not work correctly.

It produces NaN's in situations where AMD and Nvidia cards don't have such issues.
In this particular case the line of code affected is line 21 in blocklightColors.glsl (in the attached shaderpacks.zip)

vec3 torchBlockSpecialLightColor = mix(vec3(2.0, 0.87, 0.27), fireSpecialLightColor, 0.75);

Rudimentary investegation:

Doing this will fix the issue and make it behave as expected. This fix makes zero sense however as clamp(0.75, 0.0, 1.0) should do absolutely nothing to the 0.75

vec3 torchBlockSpecialLightColor = mix(vec3(2.0, 0.87, 0.27), fireSpecialLightColor, clamp01(0.75)); //this works as expected

clamp01 is just a simple function that looks like this (it's in commonFunctions.glsl)

float clamp01(float x) { return clamp(x, 0.0, 1.0); }

What makes even less sense is that, not using the function and directly doing clamp(0.75, 0.0, 1.0) results in different broken behaviour (also a NaN but it doesn't spread over the entire screen anymore)

vec3 torchBlockSpecialLightColor = mix(vec3(2.0, 0.87, 0.27), fireSpecialLightColor, clamp(0.75, 0.0, 1.0)); //this is broken

This makes precisely zero sense as clamp01 is just clamp(x , 0.0, 1.0) so there should be absolutely no difference between the two.

The suspected culprit is malfunctioning constant folding.

Reproduction Steps

Preparation:
- Install Iris & Sodium for 1.21.4 : https://github.com/IrisShaders/Iris/blob/multiloader-new/docs/guide.md
- Unpack the shaderpacks.zip inside the (...)\AppData\Roaming\.minecraft folder
- Unpack the Driver Bug Test.zip inside the \AppData\Roaming\.minecraft\saves folder
- Go into options>video settings>shaderpacks and select "complementary unbound 5.4 + euphoria patches 1.5.2"
- Press apply

Reproduction:

- Open the Driver Bug Test world in-game
- Step on the pressure plate that says "NaN apocalypse"
- Press the button to place a torch

Re: OpenGL mix() function not working correctly with Minecraft shaders.

karen_intel — Tue, 11 Feb 2025 22:24:02 GMT

@djmrFunnyMan thanks for splitting the cases.
I'll check this one too

Stay tuned

Karen

Re: OpenGL mix() function not working correctly with Minecraft shaders.

Arturo_Intel — Thu, 09 Jul 2026 17:19:15 GMT

@djmrFunnyMan

Hello,

The official update status for this case can be found here:
https://www.intel.com/content/www/us/en/support/articles/000102994.html

The team is working actively on this case

Will keep this case open for any new questions arise,

--Arturo

Re: OpenGL mix() function not working correctly with Minecraft shaders.

djmrFunnyMan — Thu, 09 Jul 2026 22:14:41 GMT

There is a big misunderstanding in the support article.

The direct call is what miscompiles. Using the helper function is a workaround to produce correct results.

Basically you mislabeled the malfunctioning and functioning examples (should be the opposite of how it is now).

Re: OpenGL mix() function not working correctly with Minecraft shaders.

djmrFunnyMan — Thu, 09 Jul 2026 22:18:34 GMT

@Arturo_Intel Please check my reply above (I forgot to ping you)

Re: OpenGL mix() function not working correctly with Minecraft shaders.

Arturo_Intel — Thu, 09 Jul 2026 22:37:28 GMT

Ow! Thanks for heads up!

I will change this ASAP.

-r2

Re: OpenGL mix() function not working correctly with Minecraft shaders.

djmrFunnyMan — Sat, 11 Jul 2026 18:53:14 GMT

@Arturo_Intel I have found another workaround.

vec3 torchBlockSpecialLightColor = mix(vec3(2.0, 0.87, 0.27), fireSpecialLightColor, 0.75); //this outputs a NaN

uniform float value = 0.75; vec3 torchBlockSpecialLightColor = mix(vec3(2.0, 0.87, 0.27), fireSpecialLightColor, value); //this outputs the correct result

This tells me that the driver can be tricked into behaving correctly by changing how the float is provided, and using helper functions is not the only way of doing so.