vertex_format_add_custom

This function adds a custom data type for specific vertex format attributes as part of the new vertex format being created.

The available values to use are defined by the data type constant that you choose, listed below: 

The use that these constants will be put to also needs to be defined so that the values can be "bound" properly within the shader being created. This is necessary due to the fact that DirectX and OpenGL have different requirements so if you don't bind them properly, they won't come through right in the shader. The available usage constants that you can choose are listed below and those you use will depend on the specifics of the shader being created: 

There are some important things to note when using custom formats like these:

• The vertex_format_add_custom() function only supports vertex_usage_position, vertex_usage_colour, vertex_usage_normal and vertex_usage_textcoord when using GLSL shaders. These will map to the shader attributes in_Position, in_Colour[0 - ...], in_Normal respectively (anything that is not one of these three attributes - e.g.: texture coordinates - can be mapped to any attribute you define).
• In general you should use vertex_usage_textcoord for all extra parameters where possible, as types like vertex_usage_blendweight and vertex_usage_tangent are close to deprecated in most shader languages, and probably won't convert properly. Instead use vec, vec2, vec3 or vec4 types vertex_usage_textcoord and everything should work fine.
• GLSL ES does not support integer attributes, so passing in ivec4's does not work (this type is usually used when passing in vertex_usage_blendindices). What you need to do is pass in texture coordinates and then in the shader, convert them to ivec4 like this:

attribute vec3 in_Position;
attribute vec4 in_BlendIndices;
attribute vec4 in_BlendWeights;

varying vec4 v_vColour;
varying mat4 v_mat;

void main()
{
    gl_Position = gm_Matrices[MATRIX_WORLD_VIEW_PROJECTION] * vec4( in_Position.xyz, 1.0);
    v_vColour = in_BlendWeights;
     ivec4 t = ivec4(in_BlendIndices);
     v_mat = gm_Matrices[ t.x ];
}

• Blend weights are usually stored in an array and then accessed using blend indices, but you can see here that instead of defining in_BlendIndices as an ivec4 attribute, it's a vec4, then cast to an ivec4 in the code. This is then used to index the array created using the gm_Matrix (you can only access an array using an INT value - not a float).

Syntax:

vertex_format_add_custom(type, usage);

Returns:

N/A

Example:

vertex_format_begin();
vertex_format_add_texcoord();
vertex_format_add_custom(vertex_type_float3, vertex_usage_position);
my_format = vertex_format_end();

The above code creates a new vertex format with just texture and 3 custom floating point values for position. It then stores the format in the variable my_format.