From 6c922f72a819e6083ffc4b6a452c2493c9170e63 Mon Sep 17 00:00:00 2001
From: Alexander Nadeau <wareya@gmail.com>
Date: Wed, 30 Apr 2025 08:12:08 -0400
Subject: [PATCH] Fix text distortion on mobile devices/browsers with `glow`
 backend (#6893)

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Did not test on platforms other than my phone, but I can't imagine it
causing problems. AFAIK if highp isn't supported then `precision highp
float;` needs to still not cause the program to fail to link/compile or
anything; it should just silently use some other precision.

* Fixes https://github.com/emilk/egui/issues/4268 for me but I only
tested it on a native Android app and I don't know whether backends
other than glow are affected.
* [x] I have followed the instructions in the PR template (but the
change is trivial so I'm just doing it from the master branch)

Before:

![image](https://github.com/user-attachments/assets/9f449749-5a48-4e9c-aef0-7a8ac3912eb6)

After:

![image](https://github.com/user-attachments/assets/544e5977-13e0-411a-bccf-b15a15289e28)
---
 crates/egui_glow/src/shader/fragment.glsl | 8 +++++++-
 crates/egui_glow/src/shader/vertex.glsl   | 8 +++++++-
 2 files changed, 14 insertions(+), 2 deletions(-)

diff --git a/crates/egui_glow/src/shader/fragment.glsl b/crates/egui_glow/src/shader/fragment.glsl
index 30da2809..f2792ed0 100644
--- a/crates/egui_glow/src/shader/fragment.glsl
+++ b/crates/egui_glow/src/shader/fragment.glsl
@@ -1,5 +1,11 @@
 #ifdef GL_ES
-    precision mediump float;
+    // To avoid weird distortion issues when rendering text etc, we want highp if possible.
+    // But apparently some devices don't support it, so we have to check first.
+    #if defined(GL_FRAGMENT_PRECISION_HIGH) && GL_FRAGMENT_PRECISION_HIGH == 1
+        precision highp float;
+    #else
+        precision mediump float;
+    #endif
 #endif
 
 uniform sampler2D u_sampler;
diff --git a/crates/egui_glow/src/shader/vertex.glsl b/crates/egui_glow/src/shader/vertex.glsl
index fff31463..0c6e9d23 100644
--- a/crates/egui_glow/src/shader/vertex.glsl
+++ b/crates/egui_glow/src/shader/vertex.glsl
@@ -9,7 +9,13 @@
 #endif
 
 #ifdef GL_ES
-    precision mediump float;
+    // To avoid weird distortion issues when rendering text etc, we want highp if possible.
+    // But apparently some devices don't support it, so we have to check first.
+    #if defined(GL_FRAGMENT_PRECISION_HIGH) && GL_FRAGMENT_PRECISION_HIGH == 1
+        precision highp float;
+    #else
+        precision mediump float;
+    #endif
 #endif
 
 uniform vec2 u_screen_size;