Lighting not affecting model

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I have a simple, image-on-plane model in a bare bones world file with a light. The light does not seem to have any effect on the model. It always appears as if it's being very dimly lit.

Am I missing something important about getting lights working correctly in Ignition Gazebo?

I have tried a variety of experiments with tweaking the light color and including the gui config for the GzScene3D plugin's ambient lighting settings. Nothing I do seems to change the way this model is rendered.

My world file looks like this:

<?xml version="1.0"?>
<sdf version="1.7">
    <world name="training_world">
         <light type="directional" name="sun">
            <pose>0 0 1 0 0 0</pose>
            <diffuse>1 1 1 1</diffuse>
            <direction>0 0 -1</direction>
        </light>
        <include>
            <uri>model://floor_mat</uri>
            <static>true</static>
        </include>
    </world>
</sdf>

The model just imports a wavefront mesh file with a jpg texture.

<?xml version='1.0'?>
<sdf version='1.7'>
    <model name='Floor Mat'>
        <link name='link'>
            <visual name='visual'>
                <pose>0 0 0 3.141592 0 0</pose>
                <geometry>
                    <mesh>
                        <uri>model://floor_mat/meshes/floor_mat_model.obj</uri>
                    </mesh>
                </geometry>
            </visual>
        </link>
    </model>
</sdf>

floor_mat_model.obj

mtllib floor_mat_model.mtl
v -0.5715  1.0164 0.0
v  0.5715  1.0164 0.0
v  0.5715 -1.0164 0.0
v -0.5715 -1.0164 0.0
vt 1.000000 0.000000
vt 1.000000 1.000000
vt 0.000000 1.000000
vt 0.000000 0.000000
vn 0.0000 0.0000 1.0000
usemtl floor_mat_material
f 1/1/1 2/2/1 3/3/1 4/4/1

floor_mat_model.mtl

newmtl floor_mat_material
Kd 1 1 1
map_Kd ../materials/textures/fll_mat.jpg