VertexColors.md

Exporting Vertex Colors

Introduction

It is possible to export the vertex color properties of objects in Blender.

Creating Vertex Colors in Blender

1. Select the object you want to create Vertex Colors for

2. (Blender version < 3.2) Go the the Object Data Properties section and expand the Vertex Colors section
   

3. (Blender version >= 3.2) Go the the Object Data Properties section and expand the Color Attributes section
   

4. Press the + button to create the Vertex Color data

5. (Blender version < 3.2)
   

6. (Blender version >= 3.2) You have to select Face Corner and Byte Color so that the Color data is compatible with the exporter
   

7. Now that we have created the data block, it is time to fill it with something useful. Change the mode to Vertex Paint and paint the vertices as you please.
   

8. To see the results in Blender you have to create a Material that takes as input the Vertex Colors. Go to the Shading tab and create a Material as follows:
   

NOTE: In Blender version >= 3.2 the input node is called Color Attribute

9. One last step is to modify the Material so that it is able to render transparency with EEVEE. Go to the Material Properties section and set Blend Mode: Alpha Hashed and Shadow Mode: None for best results
   

About exporting and importing into Ogre

The exported vertex color property shows in the resulting mesh XML as follows:

...
    <sharedgeometry vertexcount="65">
        <vertexbuffer colours_diffuse="True" normals="true" positions="true" tangent_dimensions="0" tangents="False" texture_coords="1">
            <vertex>
                <position x="0.000000" y="-1.000000" z="-1.000000"/>
                <normal x="0.000000" y="0.447214" z="-0.894427"/>
                <colour_diffuse value="1.000000 1.000000 1.000000 1.000000"/>
                <texcoord u="0.250000" v="0.510000"/>
            </vertex>
...

The option colours_diffuse="True" is an indication to OGRE and also to the blender2ogre importer that the mesh has vertex colors. The value shows as <colour_diffuse value="1.000000 1.000000 1.000000 1.000000"/> as a float4 with RGB values for color and a fourth value for the alpha.

To view the results in OGRE, the RTSS follows the normal diffuse colour flow, which can track vertex colours and contain alpha. See (vertexcolour): https://ogrecave.github.io/ogre/api/latest/_material-_scripts.html#autotoc_md128

If you're using your own shaders, then the vertex colour is passed to the vertex shader as the VES_COLOUR (5th element) and format: Colour, typically VET_UBYTE4. https://ogrecave.github.io/ogre/api/latest/group___render_system.html#gac7ecb5ad110f918f709b3c5d5cbae655

Minimal example vertex_color.vs:

#version 330 core

layout (location = 0) in vec4 vertex;
layout (location = 2) in vec3 normal;
layout (location = 3) in vec4 colour;

out vec3 FragPos;
out vec3 Normal;
out vec4 vertexColor;

uniform mat4 world;
uniform mat4 worldViewProj;

void main()
{
    FragPos = vec3(world * vertex);
    Normal = normal;
    vertexColor = colour;
    
    gl_Position = worldViewProj * vertex;
}

Minimal example vertex_color.fs:

#version 330 core

in vec3 FragPos;
in vec4 vertexColor;
in vec3 vertexNormal;

out vec4 FragColor;

uniform vec3 lightPos; 
uniform vec3 lightColor;
uniform vec3 objectColor;

void main()
{
    // ambient
    float ambientStrength = 0.3;
    vec3 ambient = ambientStrength * lightColor;

    // diffuse 
    vec3 norm = normalize(vertexNormal);
    vec3 lightDir = normalize(lightPos - FragPos);
    float diff = max(dot(norm, lightDir), 0.0);
    vec3 diffuse = diff * lightColor;
 
    vec3 result = (ambient + diffuse) * vec3(vertexColor);
    FragColor = vec4(result, vertexColor.a);
}

Program:

vertex_program Vertex-ColorVS glsl 
{
    source "vertex_color.vs"

    default_params
    {
        param_named_auto world world_matrix
        param_named_auto worldViewProj worldviewproj_matrix
    }
}

fragment_program Vertex-ColorFS glsl 
{
    source "vertex_color.fs"

    default_params
    {
        param_named_auto lightPos light_position 0
        param_named lightColor float3 1.0 1.0 1.0
    }
}

Material:

material VertexColor {
    receive_shadows on
    technique {
        pass {
            lighting on
            ambient 0.8 0.8 0.8 1.0
            diffuse 0.64 0.64 0.64 1.0
            specular 0.5 0.5 0.5 1.0 12.5
            emissive 0.0 0.0 0.0 1.0

            //scene_blend one zero 

            vertex_program_ref Vertex-ColorVS 
            {
            }

            fragment_program_ref Vertex-ColorFS
            {
            }
        }
    }
}

Troubleshooting

Some tips for troubleshooting:

• For Blender versions >= 3.2 make sure that the color data is set to Face Corner and Byte Color.
• Beware of selecting an object and directly changing the mode to Vertex Paint because the color data is automatically created.