Material and environment

Xoppa edited this page Apr 27, 2015 · 14 revisions

In practice, when rendering, you are specifying what (the shape) to render and how (the material) to render. The shape is specified using the Mesh (or more commonly the MeshPart), which defines the vertices attributes for the shader. The material is most commonly used to specify the uniform values for the shader.

Uniforms can be grouped into model specific (e.g. the texture applied or whether or not to use blending) and environmental uniforms (e.g. the lights being applied or an environment cubemap). Likewise the 3D api allows you to specify a material and environment.

Materials

Materials are model (or modelinstance) specific. You can access them by index model.materials.get(0), by name model.getMaterial("material3") or by nodepart model.nodes.get(0).parts(0).material. Materials are copied when creating a ModelInstance, meaning that changing the material of a ModelInstance will not affect the original Model or other ModelInstances.

The Material class extends the Attributes class, see below for more information about Attributes.

Environment

An Environment contains the uniform values specific for a location. For example, the lights are part of the Environment. Simple applications might use only Environment, while more complex applications might use multiple environments depending on the location of a ModelInstance. A ModelInstance (or Renderable) can only contain one Environment though.

The Environment class extends the Attributes class, see below for more information about Attributes.

Lights

Since version 1.5.7 lights are moved to attributes, which means that you can attach a light to either an environment or a material. Adding a light to an environment can still be done using the environment.add(light) method. However, you can also use the DirectionalLightsAttribute, PointLightsAttribute and SpotLightsAttribute attributes (see below). Each of these attributes has an array which you can use to attach one or more lights to it. Note however that you typically can only use one of both. If you add a light to the PointLightsAttribute of the environment and then add another light to the PointLightsAttribute of the material, then the DefaultShader will ignore the point light(s) added to the environment. Lights are always used by reference.

Lights should be sorted by importance. Usually this means that lights should be sorted on distance. The DefaultShader for example by default (configurable) only uses the first five point lights for shader lighting. Any remaining lights will be added to an ambient cubemap which is much less accurate.

Attributes

Both the Environment and Material classes extend the Attributes the class. Most commonly, the Attributes class is used to specify uniform values. For example a TextureAttribute can be used to specify an uniform to bound for a shader. However, attributes don't have to be uniforms, for example DepthTestAttribute is used to alter the opengl state and doesn't set an uniform.

The Attributes class is most comparable with a Set. It can contain at most one value for each attribute, just like an uniform can only be set to one value. Theoretically both the Material and Environment can contain the same attribute, the actual behavior in this scenario depends on the shader used, but in most cases the Materials attribute will be used instead of the Environment attribute.

Attribute type

Every attribute has a type long value, which is a bitmask used to identify the attribute. Therefor a complete material or environment can be represented with a single long, where each bit represents an attribute. Some attribute classes are dedicated to a single type value (bit). Others can be used for multiple type values (bits), in which case you must specify the type on construction. For example:

Attribute attribute = new ColorAttribute(ColorAttribute.Diffuse, Color.RED);

note that the ColorAttribute class contains a convenience method to do the same:

Attribute attribute = ColorAttribute.createDiffuse(Color.RED);

Using attributes

The most common actions for attributes are set, has, remove, and get.

You can use the set method to add or change an attribute. If there is already an attribute of the same type if will be first removed, after which the new attribute is added. For example: material.set(FloatAttribute.createAlphaTest(0.25f));

Using the has method it is possible to check if a specific attribute type is set. For example: material.has(FloatAttribute.AlphaTest);. It is also possible to check for multiple attributes, for example: material.has(FloatAttribute.AlphaTest | ColorAttribute.Diffuse);. In which case the has method will only return true if all attributes are set. Note that because the has method uses a bitwise check it is quite fast and can be used prior to e.g. a call to remove to ensure the attribute is actually set.

With the remove method you can remove an attribute of a specific type, for example: material.remove(FloatAttribute.AlphaTest);. You can also remove multiple attributes at once, for example: material.remove(FloatAttribute.AlphaTest | ColorAttribute.Diffuse);.

The get method can be used to fetch an attribute of a specific type. For example: material.get(FloatAttribute.AlphaTest);. If the attribute isn't set the get method will return null. Because the type implies the class, you can safely cast the result without checking: (FloatAttribute)material.get(FloatAttribute.AlphaTest);. For convenience there's also a template method: material.get(FloatAttribute.class, FloatAttribute.AlphaTest);.

Besides that you can also clear (to remove all attributes), iterate (it implements Iterable<Attribute>) and compare (it implements Comparator<Attribute>, however the same method provides additional options) attributes. The getMask() method provides access to the mask containing all attributes, for example material.getMask() & FloatAttribute.AlphaTest == FloatAttribute.AlphaTest is the same as material.has(FloatAttribute.AlphaTest).

Custom attribute types

It is possible to use a standard attribute class for a new custom type. For example when you want to use the ColorAttribute class to specify a custom type of color. There are three things you must consider in that case:

  1. Name your attribute type. Each attribute type must have an unique alias (name). You might want to (but don't have to) use the uniform name for that. The alias will also be used for debugging, e.g. when calling attribute.toString().
  2. Register your attribute type. This is to make sure there is only one attribute type for each bit. This can be done only from within the Attribute class or a subclass.
  3. Make ColorAttribute accept the custom type. The ColorAttribute class and most other attribute classes checks the type on construction, this allows you to cast attributes without having to check anything other then the type. For example (ColorAttribute)material.get(ColorAttribute.Diffuse) will always work because ColorAttribute is the only attribute accepting the ColorAttribute.Diffuse type.

Because of step 2 and 3, you must extend the Attribute class to add a custom attribute type:

public class CustomColorTypes extends ColorAttribute {
    public final static String AlbedoColorAlias = "AlbedoColor"; // step 1: name the type
    public final static long AlbedoColor = register(AlbedoColorAlias); // step 2: register the type
    static {
        Mask |= AlbedoColor; // step 3: Make ColorAttribute accept the type
    }
    /** Prevent instantiating this class */
    private CustomColorTypes() {
        super(0);
    }
}

You can then create the custom attribute type using:

Attribute attribute = new ColorAttribute(CustomColorTypes.AlbedoColor, Color.RED);

Custom attributes

It is possible to create a custom attribute, in which case the process is not much different from above. You must extend the Attribute class, register at least one type and of course add some data to pass on to the shader. For example to add an attribute to pass a double value to the shader:

public class DoubleAttribute extends Attribute {
    public final static String MyDouble1Alias = "myDouble1";
    public final static long MyDouble1 = register(MyDouble1Alias);
    public final static String MyDouble2Alias = "myDouble2";
    public final static long MyDouble2 = register(MyDouble2Alias);
    protected static long Mask = MyDouble1 | MyDouble2;
    /** Method to check whether the specified type is a valid DoubleAttribute type */
    public static Boolean is(final long type) {
        return (type & Mask) != 0;
    }

    public double value;

    public DoubleAttribute (final long type) {
        super(type);
        if (!is(type))
            throw new GdxRuntimeException("Invalid type specified");
    }

    public DoubleAttribute (final long type, final double value) {
        this(type);
        this.value = value;
    }

    /** copy constructor */
    public DoubleAttribute (DoubleAttribute other) {
        this(other.type, other.value);
    }

    @Override
    public Attribute copy () {
        return new DoubleAttribute(this);
    }

    @Override
    public int hashCode () {
        final int prime = /* pick a prime number and use it here */;
        final long v = NumberUtils.doubleToLongBits(value);
        return prime * super.hashCode() + (int)(v^(v>>>32));
    }

    @Override
    public int compareTo (Attribute o) {
        if (type != o.type) return type < o.type ? -1 : 1;
        double otherValue = ((DoubleAttribute)o).value;
        return value == otherValue ? 0 : (value < otherValue ? -1 : 1);
    }
}

Of course MyDouble1Alias, MyDouble1, "myDouble1", MyDouble2Alias, MyDouble2 and "myDouble2" should be replaced by a more meaningful description.

Note that the copy() method is for example called when creating a ModelInstance of a Model. It should return an identical instance of the attribute which can be modified independently of the attribute being copied. While not required, a copy constructor is typically a good method to implement this.

The hashCode() method should be implemented because it is used for comparing attributes and materials. For example, two materials are considered to be the same if they contain the same attributes (types) and the equals() method returns true for each pair of attribute types. By default, the equals() method of the Attribute class compares the hashCode() of both attributes for this.

The compareTo(Attribute) method must be implemented for sorting render calls based on the material. This implementation should typically always start with the line: if (type != o.type) return type < o.type ? -1 : 1; to assure that it's comparing attributes of the same type.

Attribute classes should be kept small and self contained, therefore it is best to always directly extend the Attribute class. Try to avoid extending a subclass of the Attribute class to add additional information.

Available attributes

Like stated above its possible to create custom attributes. However, there are a few attributes already included, which are listed below.

BlendingAttribute

By default the 3D api assumes everything is opaque. The BlendingAttribute is most commonly used for materials (in case of environment it will change the default behavior) and can be used to specify that the material is or is not blended. The BlendingAttribute doesn't require you to specify a type on construction, its type is always BlendingAttribute.Type, unless it's extended. It contains four properties which can be specified:

  • blended indicates whether or not the material should be treated as blended. This is primarily used for sorting, for example opaque objects are drawn prior to transparent objects.
  • sourceFunction OpenGL enum which specifies how the (incoming) red, green, blue, and alpha source blending factors are computed, by default it is set to GL_SRC_ALPHA.
  • destFunction OpenGL enum which specifies how the (existing) red, green, blue, and alpha destination blending factors are computed, by default it is set to GL_ONE_MINUS_SRC_ALPHA. For additive blending you might want to set it to GL_ONE.
  • opacity The amount of opacity (the source alpha value), ranging from 0 (fully transparent) to 1 (fully opaque).

ColorAttribute

The ColorAttribute allows you to pass a color to the shader. For that it only contains one property: .color. You can set the color during construction (it will be set by value) or using the .color.set(...) method. The ColorAttribute requires an attribute type to be specified, by default the following types are available:

  • ColorAttribute.Diffuse
  • ColorAttribute.Specular
  • ColorAttribute.Ambient
  • ColorAttribute.Emissive
  • ColorAttribute.Reflection
  • ColorAttribute.AmbientLight
  • ColorAttribute.Fog

Where the latter two are most commonly used for Environment, while the others or commonly used for Material.

CubemapAttribute

To pass a Cubemap to the shader the CubemapAttribute can be used. It's value is the textureDescription member which can be used to specify the cubemap along with other texture related values. The CubemapAttribute requires an attribute type to be specified, by default the CubemapAttribute.EnvironmentMap is the only valid type.

DepthTestAttribute

Just like the BlendingAttribute, does the DepthTestAttribute not require an attribute type. It is always DepthTestAttribute.Type. The DepthTestAttribute can be used to specify depth testing and writing, using the following properties:

  • depthFunc The depth test function, or 0 (or GL_NONE) to disable depth test, by default it is GL20.GL_LEQUAL.
  • depthRangeNear Mapping of near clipping plane to window coordinates, by default 0.0
  • depthRangeFar Mapping of far clipping plane to window coordinates, by default 1.0
  • depthMask Whether or not to write to the depth buffer, enabled by default.

DirectionalLightsAttribute

The DirectionalLightsAttribute does not require an attribute type. It is always DirectionalLightsAttribute.Type. The DirectionalLightsAttribute can be used to specify an array of DirectionalLight instances, using the following property:

  • lights The array of lights, should be sorted on importance.

FloatAttribute

To pass a single floating point value to the shader, the FloatAttribute can be used. The value can be specified on construction or using the .value member. The FloatAttribute requires an attribute type, which by default can be:

  • FloatAttribute.Shininess Used for specular lighting.
  • FloatAttribute.AlphaTest Used to discard pixels when the alpha value is equal or below the specified value.

IntAttribute

Similar to the FloatAttribute class, the IntAttribute allows you to pass an integer value to the shader. Likewise the .value member can be used or the value can be set on construction. The IntAttribute requires an attribute type, which by default can be:

  • IntAttribute.CullFace OpenGL enum to specify face culling, either GL_NONE (no culling), GL_FRONT (only render back faces) or GL_BACK (only render front faces). The default depends on the shader, the default shader uses GL_BACK by default.

PointLightsAttribute

The PointLightsAttribute does not require an attribute type. It is always PointLightsAttribute.Type. The PointLightsAttribute can be used to specify an array of PointLight instances, using the following property:

  • lights The array of lights, should be sorted on importance.

SpotLightsAttribute

Not supported by the default shader. The SpotLightsAttribute does not require an attribute type. It is always SpotLightsAttribute.Type. The SpotLightsAttribute can be used to specify an array of SpotLight instances, using the following property:

  • lights The array of lights, should be sorted on importance.

TextureAttribute

TextureAttribute can be used to pass a Texture to the shader. Just like the CubemapAttribute it has a textureDescription member which allows you to set the Texture amongst some texture related values like repeat and filter. Additionally it contains the offsetU, offsetV, scaleU and scaleY members, which can be used to specify the region (texture coordinates transformation) of the texture to use. It also has an uvIndex member (defaults to 0) which can be used to specify which texture coordinates should be used. Note that the default shader currently ignores this uvIndex member and always uses the first texture coordinates.

The TextureAttribute requires an attribute type, which by default can be one of the following:

  • TextureAttribute.Diffuse
  • TextureAttribute.Specular
  • TextureAttribute.Bump
  • TextureAttribute.Normal

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