Apply slight z-offsets to distance field glyphs

When rendering distance field text, z-fighting may occur between
adjacent glyphs. To prevent this, we apply a very slight Z offset
to each rendered glyph.

Change-Id: I19f9808267547ae9521757c6022b4f65c1ca4ba5
Reviewed-by: Sean Harmer <sean.harmer@kdab.com>

3 files changed, 11 insertions, 10 deletions

diff --git a/src/extras/shaders/gl3/distancefieldtext.frag b/src/extras/shaders/gl3/distancefieldtext.frag
index 4f0c9cac0..c53f3df55 100644
--- a/src/extras/shaders/gl3/distancefieldtext.frag
+++ b/src/extras/shaders/gl3/distancefieldtext.frag
@@ -1,4 +1,4 @@
-#version 150 core
+#version 130
 
 uniform sampler2D distanceFieldTexture;
 uniform float minAlpha;
@@ -6,8 +6,8 @@ uniform float maxAlpha;
 uniform float textureSize;
 uniform vec4 color;
 
-in vec3 position;
 in vec2 texCoord;
+in float zValue;
 
 out vec4 fragColor;
 
@@ -35,4 +35,5 @@ void main()
 
     float distVal = texture(distanceFieldTexture, texCoord).r;
     fragColor = color * smoothstep(minAlpha, maxAlpha, distVal);
+    gl_FragDepth = gl_FragCoord.z - zValue * 0.00001;
 }
diff --git a/src/extras/shaders/gl3/distancefieldtext.vert b/src/extras/shaders/gl3/distancefieldtext.vert
index 9bd2a0a90..74a48f426 100644
--- a/src/extras/shaders/gl3/distancefieldtext.vert
+++ b/src/extras/shaders/gl3/distancefieldtext.vert
@@ -1,10 +1,10 @@
-#version 150 core
+#version 130
 
 in vec3 vertexPosition;
 in vec2 vertexTexCoord;
 
-out vec3 position;
 out vec2 texCoord;
+out float zValue;
 
 uniform mat4 modelView;
 uniform mat4 mvp;
@@ -12,8 +12,8 @@ uniform mat4 mvp;
 void main()
 {
     texCoord = vertexTexCoord;
-    position = vec3(modelView * vec4(vertexPosition, 1.0));
+    zValue = vertexPosition.z;
 
-    gl_Position = mvp * vec4(vertexPosition, 1.0);
+    gl_Position = mvp * vec4(vertexPosition.xy, 0.0, 1.0);
 }
 
diff --git a/src/extras/text/qtext2dentity.cpp b/src/extras/text/qtext2dentity.cpp
index dbd4368bc..1dfd171cd 100644
--- a/src/extras/text/qtext2dentity.cpp
+++ b/src/extras/text/qtext2dentity.cpp
@@ -209,10 +209,10 @@ void QText2DEntityPrivate::setCurrentGlyphRuns(const QVector<QGlyphRun> &runs)
                 texCoords.setWidth(texCoords.width() * insideRatio);
             }
 
-            data.vertex << x1 << y1 << 0.f << texCoords.left() << texCoords.bottom();
-            data.vertex << x1 << y2 << 0.f << texCoords.left() << texCoords.top();
-            data.vertex << x2 << y1 << 0.f << texCoords.right() << texCoords.bottom();
-            data.vertex << x2 << y2 << 0.f << texCoords.right() << texCoords.top();
+            data.vertex << x1 << y1 << i << texCoords.left() << texCoords.bottom();
+            data.vertex << x1 << y2 << i << texCoords.left() << texCoords.top();
+            data.vertex << x2 << y1 << i << texCoords.right() << texCoords.bottom();
+            data.vertex << x2 << y2 << i << texCoords.right() << texCoords.top();
 
             data.index << data.vertexCount << data.vertexCount+3 << data.vertexCount+1;
             data.index << data.vertexCount << data.vertexCount+2 << data.vertexCount+3;