Add distance field rendering to OpenGL runtime

The old text rendering remains in the background and is used to render
clipped text, since that feature is not yet implemented for distance
field fonts. Building the runtime with Qt version older than 5.12.2 also
causes a fallback to the old text rendering.

Depth pass rendering also needs to be redone in the future to avoid
another full render pass.

Task-number: QT3DS-3210
Change-Id: Ib7666c437d23ae25e1872682f010df3721476a14
Reviewed-by: Tomi Korpipää <tomi.korpipaa@qt.io>

1 files changed, 54 insertions, 0 deletions

diff --git a/src/Runtime/res/effectlib/distancefieldtext_dropshadow_core.vert b/src/Runtime/res/effectlib/distancefieldtext_dropshadow_core.vert
new file mode 100644
index 00000000..51190577
--- /dev/null
+++ b/src/Runtime/res/effectlib/distancefieldtext_dropshadow_core.vert
@@ -0,0 +1,54 @@
+in vec3 vCoord;
+in vec2 tCoord;
+in vec4 textureBounds;
+
+out vec2 sampleCoord;
+out vec2 shadowSampleCoord;
+
+out vec2 alphas;
+out vec4 normalizedTextureBounds;
+
+uniform mat4 mvp;
+uniform mat4 modelView;
+uniform int textureWidth;
+uniform int textureHeight;
+uniform float fontScale;
+uniform vec2 shadowOffset;
+
+float thresholdFunc(float scale)
+{
+    float base = 0.5;
+    float baseDev = 0.065;
+    float devScaleMin = 0.15;
+    float devScaleMax = 0.3;
+    return base - ((clamp(scale, devScaleMin, devScaleMax) - devScaleMin)
+                   / (devScaleMax - devScaleMin) * -baseDev + baseDev);
+}
+
+float spreadFunc(float scale)
+{
+    return 0.06 / scale;
+}
+
+vec2 alphaRange(float scale)
+{
+    float base = thresholdFunc(scale);
+    float range = spreadFunc(scale);
+    float alphaMin = max(0.0, base - range);
+    float alphaMax = min(base + range, 1.0);
+    return vec2(alphaMin, alphaMax);
+}
+
+void main()
+{
+     float scale = fontScale * sqrt(abs(determinant(modelView)));
+     alphas = alphaRange(scale);
+
+     vec2 textureSizeMultiplier = vec2(1.0 / textureWidth, 1.0 / textureHeight);
+
+     sampleCoord = tCoord * textureSizeMultiplier;
+     shadowSampleCoord = (tCoord - shadowOffset) * textureSizeMultiplier;
+     normalizedTextureBounds = vec4(textureBounds.xy * textureSizeMultiplier,
+                                    textureBounds.zw * textureSizeMultiplier);
+     gl_Position = mvp * vec4(vCoord, 1.0);
+}