custom variables can now have their precision specified

e.g.:

    variables : [
        {
            name : vertex,
            precision : medium,
         }
    ],

docs/Materials.md.html

@@ -1308,7 +1308,12 @@
       declare a variable called `eyeDirection` you can access it in the fragment shader using
       `variable_eyeDirection`. In the vertex shader, the interpolant name is simply a member of
       the `MaterialVertexInputs` structure (`material.eyeDirection` in your example). Each
-      interpolant is of type `float4` (`vec4`) in the shaders.
+      interpolant is of type `float4` (`vec4`) in the shaders. By default the precision of the
+      interpolant is `highp` in *both* the vertex and fragment shaders.
+      An alternate syntax can be used to specify both the name and precision of the interpolant.
+      In this case the specified precision is used as-is in both fragment and vertex stages, in
+      particular if `default` is specified the default precision is used is the fragment shader
+      (`mediump`) and in the vertex shader (`highp`).
 
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ JSON
 material {
@@ -1320,7 +1325,11 @@
         }
     ],
     variables : [
-         eyeDirection
+         eyeDirection,
+         {
+            name : eyeColor,
+            precision : medium
+         }
     ],
     vertexDomain : device,
     depthWrite : false,

libs/filamat/include/filamat/MaterialBuilder.h

@@ -323,6 +323,9 @@ class UTILS_PUBLIC MaterialBuilder : public MaterialBuilderBase {
     //! Custom variables (all float4).
     MaterialBuilder& variable(Variable v, const char* name) noexcept;
 
+    MaterialBuilder& variable(Variable v, const char* name,
+            ParameterPrecision precision) noexcept;
+
     /**
      * Require a specified attribute.
      *
@@ -708,11 +711,17 @@ class UTILS_PUBLIC MaterialBuilder : public MaterialBuilderBase {
         ShaderStage stage;
     };
 
+    struct CustomVariable {
+        utils::CString name;
+        Precision precision;
+        bool hasPrecision;
+    };
+
     static constexpr size_t MATERIAL_PROPERTIES_COUNT = filament::MATERIAL_PROPERTIES_COUNT;
     using Property = filament::Property;
 
     using PropertyList = bool[MATERIAL_PROPERTIES_COUNT];
-    using VariableList = utils::CString[MATERIAL_VARIABLES_COUNT];
+    using VariableList = CustomVariable[MATERIAL_VARIABLES_COUNT];
     using OutputList = std::vector<Output>;
 
     static constexpr size_t MAX_COLOR_OUTPUT = filament::backend::MRT::MAX_SUPPORTED_RENDER_TARGET_COUNT;

libs/filamat/src/MaterialBuilder.cpp

@@ -234,7 +234,21 @@ MaterialBuilder& MaterialBuilder::variable(Variable v, const char* name) noexcep
         case Variable::CUSTOM2:
         case Variable::CUSTOM3:
             assert(size_t(v) < MATERIAL_VARIABLES_COUNT);
-            mVariables[size_t(v)] = CString(name);
+            mVariables[size_t(v)] = { CString(name), Precision::DEFAULT, false };
+            break;
+    }
+    return *this;
+}
+
+MaterialBuilder& MaterialBuilder::variable(Variable v,
+        const char* name, ParameterPrecision precision) noexcept {
+    switch (v) {
+        case Variable::CUSTOM0:
+        case Variable::CUSTOM1:
+        case Variable::CUSTOM2:
+        case Variable::CUSTOM3:
+            assert(size_t(v) < MATERIAL_VARIABLES_COUNT);
+            mVariables[size_t(v)] = { CString(name), precision, true };
             break;
     }
     return *this;
@@ -1383,7 +1397,7 @@ bool MaterialBuilder::checkMaterialLevelFeatures(MaterialInfo const& info) const
 
 bool MaterialBuilder::hasCustomVaryings() const noexcept {
     for (const auto& variable : mVariables) {
-        if (!variable.empty()) {
+        if (!variable.name.empty()) {
             return true;
         }
     }

libs/filamat/src/shaders/CodeGenerator.cpp

@@ -451,15 +451,20 @@ io::sstream& CodeGenerator::generatePostProcessMain(io::sstream& out, ShaderStag
 }
 
 io::sstream& CodeGenerator::generateVariable(io::sstream& out, ShaderStage stage,
-        const CString& name, size_t index) {
-
+        const MaterialBuilder::CustomVariable& variable, size_t index) {
+    auto const& name = variable.name;
+    const char* precisionString = getPrecisionQualifier(variable.precision);
     if (!name.empty()) {
         if (stage == ShaderStage::VERTEX) {
             out << "\n#define VARIABLE_CUSTOM" << index << " " << name.c_str() << "\n";
             out << "\n#define VARIABLE_CUSTOM_AT" << index << " variable_" << name.c_str() << "\n";
-            out << "LAYOUT_LOCATION(" << index << ") VARYING vec4 variable_" << name.c_str() << ";\n";
+            out << "LAYOUT_LOCATION(" << index << ") VARYING " << precisionString << " vec4 variable_" << name.c_str() << ";\n";
         } else if (stage == ShaderStage::FRAGMENT) {
-            out << "\nLAYOUT_LOCATION(" << index << ") VARYING highp vec4 variable_" << name.c_str() << ";\n";
+            if (!variable.hasPrecision && variable.precision == Precision::DEFAULT) {
+                // for backward compatibility
+                precisionString = "highp";
+            }
+            out << "\nLAYOUT_LOCATION(" << index << ") VARYING " << precisionString << " vec4 variable_" << name.c_str() << ";\n";
         }
     }
     return out;

libs/filamat/src/shaders/CodeGenerator.h

@@ -103,7 +103,7 @@ class UTILS_PRIVATE CodeGenerator {
 
     // generate declarations for custom interpolants
     static utils::io::sstream& generateVariable(utils::io::sstream& out, ShaderStage stage,
-            const utils::CString& name, size_t index);
+            const MaterialBuilder::CustomVariable& variable, size_t index);
 
     // generate declarations for non-custom "in" variables
     utils::io::sstream& generateShaderInputs(utils::io::sstream& out, ShaderStage type,

tools/matc/src/matc/ParametersProcessor.cpp

@@ -615,14 +615,51 @@ static bool processVariables(MaterialBuilder& builder, const JsonishValue& value
     }
 
     for (size_t i = 0; i < elements.size(); i++) {
-        auto elementValue = elements[i];
+        ParameterPrecision precision = ParameterPrecision::DEFAULT;
         MaterialBuilder::Variable v = intToVariable(i);
-        if (elementValue->getType() != JsonishValue::Type::STRING) {
+        std::string nameString;
+
+        auto elementValue = elements[i];
+        if (elementValue->getType() == JsonishValue::Type::OBJECT) {
+
+            JsonishObject const& jsonObject = *elementValue->toJsonObject();
+
+            const JsonishValue* nameValue = jsonObject.getValue("name");
+            if (!nameValue) {
+                std::cerr << "variables: entry without 'name' key." << std::endl;
+                return false;
+            }
+            if (nameValue->getType() != JsonishValue::STRING) {
+                std::cerr << "variables: name value must be STRING." << std::endl;
+                return false;
+            }
+
+            const JsonishValue* precisionValue = jsonObject.getValue("precision");
+            if (precisionValue) {
+                if (precisionValue->getType() != JsonishValue::STRING) {
+                    std::cerr << "variables: precision must be a STRING." << std::endl;
+                    return false;
+                }
+                auto precisionString = precisionValue->toJsonString();
+                if (!Enums::isValid<ParameterPrecision>(precisionString->getString())){
+                    return logEnumIssue("variables", *precisionString, Enums::map<ParameterPrecision>());
+                }
+            }
+
+            nameString = nameValue->toJsonString()->getString();
+            if (precisionValue) {
+                precision = Enums::toEnum<ParameterPrecision>(
+                        precisionValue->toJsonString()->getString());
+            }
+            builder.variable(v, nameString.c_str(), precision);
+        } else if (elementValue->getType() == JsonishValue::Type::STRING) {
+            nameString = elementValue->toJsonString()->getString();
+            builder.variable(v, nameString.c_str());
+        } else {
             std::cerr << "variables: array index " << i << " is not a STRING. found:" <<
                     JsonishValue::typeToString(elementValue->getType()) << std::endl;
             return false;
         }
-        builder.variable(v, elementValue->toJsonString()->getString().c_str());
     }
 
     return true;