GLSLShader.set keeps track of value changes, and returns True or False

so caller can tell whether shader state was actually
changed. GLVolume (for OpenGL 2.1) updated to use this feature.

fsl/fsleyes/gl/gl14/glvolume_funcs.py

@@ -106,6 +106,8 @@ def updateShaderState(self):
     
     self.shader.unload()
 
+    return True
+
 
 def preDraw(self):
     """Prepares to draw a slice from the given :class:`.GLVolume` instance. """

fsl/fsleyes/gl/gl21/glvolume_funcs.py

@@ -83,24 +83,28 @@ def updateShaderState(self):
         self.colourTexture.getCoordinateTransform())
 
     shader.load()
+
+    changed = False
     
-    shader.set('useSpline',        opts.interpolation == 'spline')
-    shader.set('imageShape',       self.image.shape[:3])
-    shader.set('clipLow',          clipLow)
-    shader.set('clipHigh',         clipHigh)
-    shader.set('texZero',          texZero)
-    shader.set('invertClip',       opts.invertClipping)
-    shader.set('useNegCmap',       opts.useNegativeCmap)
-    shader.set('imageIsClip',      opts.clipImage is None)
-    shader.set('img2CmapXform',    img2CmapXform)
-
-    shader.set('imageTexture',     0)
-    shader.set('colourTexture',    1)
-    shader.set('negColourTexture', 2)
-    shader.set('clipTexture',      3)
+    changed |= shader.set('useSpline',        opts.interpolation == 'spline')
+    changed |= shader.set('imageShape',       self.image.shape[:3])
+    changed |= shader.set('clipLow',          clipLow)
+    changed |= shader.set('clipHigh',         clipHigh)
+    changed |= shader.set('texZero',          texZero)
+    changed |= shader.set('invertClip',       opts.invertClipping)
+    changed |= shader.set('useNegCmap',       opts.useNegativeCmap)
+    changed |= shader.set('imageIsClip',      opts.clipImage is None)
+    changed |= shader.set('img2CmapXform',    img2CmapXform)
+
+    changed |= shader.set('imageTexture',     0)
+    changed |= shader.set('colourTexture',    1)
+    changed |= shader.set('negColourTexture', 2)
+    changed |= shader.set('clipTexture',      3)
 
     shader.unload()
 
+    return changed
+
 
 def preDraw(self):
     """Sets up the GL state to draw a slice from the given :class:`.GLVolume`

fsl/fsleyes/gl/glvolume.py

@@ -265,11 +265,10 @@ class GLVolume(globject.GLImageObject):
         def update(*a):
             self.notify()
 
-
         def shaderUpdate(*a):
             if self.ready():
-                fslgl.glvolume_funcs.updateShaderState(self)
-                self.notify()
+                if fslgl.glvolume_funcs.updateShaderState(self):
+                    self.notify()
         
         def colourUpdate(*a):
             self.refreshColourTextures()

fsl/fsleyes/gl/shaders/glsl/program.py

@@ -149,6 +149,13 @@ class GLSLShader(object):
                                              self.vertUniforms,
                                              self.fragUniforms)
 
+        # We cache the most recent value for
+        # every uniform. When a call to set()
+        # is made, if the value is unchanged,
+        # we skip the GL call.
+        self.values = {n : None for n in self.positions.keys()}
+
+        # Buffers for vertex attributes
         self.buffers = {}
 
         for att in self.vertAttributes:
@@ -234,7 +241,16 @@ class GLSLShader(object):
         
         
     def set(self, name, value):
-        """Sets the value for the specified GLSL ``uniform`` variable. """
+        """Sets the value for the specified GLSL ``uniform`` variable.
+
+        The ``GLSLShader`` keeps a copy of the value of every uniform, to
+        avoid unnecessary GL calls.
+
+        :returns: ``True`` if the value was changed, ``False`` otherwise.
+        """
+
+        if self.values[name] == value:
+            return False
 
         vPos  = self.positions[name]
         vType = self.types[    name]
@@ -250,6 +266,8 @@ class GLSLShader(object):
 
         setfunc(vPos, value)
 
+        return True
+
 
     def setAtt(self, name, value, divisor=None):
         """Sets the value for the specified GLSL ``attribute`` variable.