diff --git a/doc/contributions.txt b/doc/contributions.txt
index 9368781c9a6cc27e81d9a2a3b4830da743a261eb..4cabb84b1034af760667cf16d4f0d9384e2edc93 100755
--- a/doc/contributions.txt
+++ b/doc/contributions.txt
@@ -282,6 +282,7 @@ Beq Janus
 	SL-11300
 	SL-15709
 	SL-16021
+	SL-18637
 Beth Walcher
 Bezilon Kasei
 Biancaluce Robbiani
diff --git a/indra/newview/app_settings/shaders/class1/deferred/materialF.glsl b/indra/newview/app_settings/shaders/class1/deferred/materialF.glsl
index b26194f278823da1da30ec16972e52f09e3ee60c..b30d7655dbe468f713b44f0f6795ade71e8149fb 100644
--- a/indra/newview/app_settings/shaders/class1/deferred/materialF.glsl
+++ b/indra/newview/app_settings/shaders/class1/deferred/materialF.glsl
@@ -202,7 +202,7 @@ VARYING vec2 vary_texcoord2;
 uniform float env_intensity;
 uniform vec4 specular_color;  // specular color RGB and specular exponent (glossiness) in alpha
 
-#ifdef HAS_ALPHA_MASK
+#if (DIFFUSE_ALPHA_MODE == DIFFUSE_ALPHA_MODE_MASK)
 uniform float minimum_alpha;
 #endif
 
@@ -227,12 +227,11 @@ void main()
     vec4 diffcol = texture2D(diffuseMap, vary_texcoord0.xy);
 	diffcol.rgb *= vertex_color.rgb;
 
-#ifdef HAS_ALPHA_MASK
-#if DIFFUSE_ALPHA_MODE == DIFFUSE_ALPHA_MODE_BLEND
-    if (diffcol.a*vertex_color.a < minimum_alpha)
-#else
-    if (diffcol.a < minimum_alpha)
-#endif
+#if (DIFFUSE_ALPHA_MODE == DIFFUSE_ALPHA_MODE_MASK)
+
+    // Comparing floats cast from 8-bit values, produces acne right at the 8-bit transition points
+    float bias = 0.001953125; // 1/512, or half an 8-bit quantization (SL-18637)
+    if (diffcol.a < minimum_alpha-bias)
     {
         discard;
     }