Allow glyph cache textures to be created at full size.

On embedded hardware, texture resizes can be quite slow. The glyph
cache currently often needs to resize when new glyphs are added, and
glyph additions are very frequent when using a language with a large
set of glyphs (such as Chinese).

The current glyph cache design minimizes the amount of memory used.
When QSG_PREFER_FULLSIZE_GLYPHCACHE_TEXTURES is set, and we are using
a font with a large number of glyphs, we can instead allocate max-sized
textures. This leads to significantly less time when inserting glyphs
into the cache (often incurred over a span of time), at the cost of
higher initial memory and creation times (often incurred at
application startup).

Change-Id: Id1021b9d213e5f8635c4197b624474f28c6f44ff
Task-number: QTBUG-29264
Reviewed-by: Robin Burchell <robin.burchell@viroteck.net>
Reviewed-by: Gunnar Sletta <gunnar@sletta.org>
Reviewed-by: Yoann Lopes <yoann.lopes@theqtcompany.com>

2 files changed, 20 insertions, 3 deletions

diff --git a/src/quick/scenegraph/qsgdefaultdistancefieldglyphcache.cpp b/src/quick/scenegraph/qsgdefaultdistancefieldglyphcache.cpp
index 9ef0d50dfc..16745c18d5 100644
--- a/src/quick/scenegraph/qsgdefaultdistancefieldglyphcache.cpp
+++ b/src/quick/scenegraph/qsgdefaultdistancefieldglyphcache.cpp
@@ -47,6 +47,7 @@
 QT_BEGIN_NAMESPACE
 
 DEFINE_BOOL_CONFIG_OPTION(qmlUseGlyphCacheWorkaround, QML_USE_GLYPHCACHE_WORKAROUND)
+DEFINE_BOOL_CONFIG_OPTION(qsgPreferFullSizeGlyphCacheTextures, QSG_PREFER_FULLSIZE_GLYPHCACHE_TEXTURES)
 
 #if !defined(QSG_DEFAULT_DISTANCEFIELD_GLYPH_CACHE_PADDING)
 #  define QSG_DEFAULT_DISTANCEFIELD_GLYPH_CACHE_PADDING 2
@@ -491,6 +492,17 @@ bool QSGDefaultDistanceFieldGlyphCache::useTextureUploadWorkaround() const
     return useWorkaround;
 }
 
+bool QSGDefaultDistanceFieldGlyphCache::createFullSizeTextures() const
+{
+    static bool set = false;
+    static bool fullSize = false;
+    if (!set) {
+        fullSize = qsgPreferFullSizeGlyphCacheTextures() && glyphCount() > QT_DISTANCEFIELD_HIGHGLYPHCOUNT;
+        set = true;
+    }
+    return fullSize;
+}
+
 int QSGDefaultDistanceFieldGlyphCache::maxTextureSize() const
 {
     if (!m_maxTextureSize)
diff --git a/src/quick/scenegraph/qsgdefaultdistancefieldglyphcache_p.h b/src/quick/scenegraph/qsgdefaultdistancefieldglyphcache_p.h
index 3b64eadc6f..cffe02330a 100644
--- a/src/quick/scenegraph/qsgdefaultdistancefieldglyphcache_p.h
+++ b/src/quick/scenegraph/qsgdefaultdistancefieldglyphcache_p.h
@@ -62,6 +62,7 @@ public:
 
     bool useTextureResizeWorkaround() const;
     bool useTextureUploadWorkaround() const;
+    bool createFullSizeTextures() const;
     int maxTextureSize() const;
 
     void setMaxTextureCount(int max) { m_maxTextureCount = max; }
@@ -75,7 +76,7 @@ private:
         QDistanceField image;
         int padding;
 
-        TextureInfo() : texture(0), padding(-1)
+        TextureInfo(const QRect &preallocRect = QRect()) : texture(0), allocatedArea(preallocRect), padding(-1)
         { }
     };
 
@@ -84,8 +85,12 @@ private:
 
     TextureInfo *textureInfo(int index)
     {
-        for (int i = m_textures.count(); i <= index; ++i)
-            m_textures.append(TextureInfo());
+        for (int i = m_textures.count(); i <= index; ++i) {
+            if (createFullSizeTextures())
+                m_textures.append(QRect(0, 0, maxTextureSize(), maxTextureSize()));
+            else
+                m_textures.append(TextureInfo());
+        }
 
         return &m_textures[index];
     }