There appears to be a software defect in the SketchUp API. Loading the textures into TextureWriter and then attempting to access their filenames using TextureWriter.filename(index) returns only empty strings. However, if you call TextureWriter.write_all beforehand:

# Display texture names
def display_tex_names
    $tw = Sketchup.create_texture_writer
    load_textures(Sketchup.active_model.entities)

    $tw.write_all("D;", false)
    
    tex_index = 1
    while (tex_index <= $tw.count)
        UI.messagebox($tw.filename(tex_index))
        tex_index += 1        
    end

    Sketchup.active_model.commit_operation
end

then the filenames are returned properly.

Confusing matters further, the TextureWriterindices are one-based rather than the expected zero-based array.

@tig said:

What is load_textures(...) ?
Why not simply use Didier's & Others very-fast-texture-writer code as an example.
http://sketchucation.com/forums/viewtopic.php?p=299463#p299463
Rather than iterate all of the model.entities you can iterate the materials, ignore ones without textures and apply each material to an empty group that you use as the 'source' in the tw ?

load_textures is a recursive function that iterates over all entities:

# Load textures (recursive))
def load_textures( entities )
    entities.each do |entity|    
        if entity.is_a?(Sketchup;;Face)
            $tw.load(entity, true)
            $tw.load(entity, false)
        elsif entity.is_a?(Sketchup;;Group) ||
                entity.is_a?(Sketchup;;ComponentInstance)
            if entity.is_a?(Sketchup;;Group) 
                definition = entity.entities.parent
            else
                definition = entity.definition
            end
            load_textures(definition.entities)     
        end
    end
end

I did not post it because it (presumably) works.

The problem with very-fast-texture-writer(for me) is that it returns the textures associated with all of the materials, including those not actually used by the entities. I would need to delete these unreferenced textures for my application, which means yet another pass through all the entities.

This still begs the question, however, of why TextureWriter.filename is returning zero-length strings. (I am an expert C++ developer but a neophyte Ruby programmer, so I am not ruling out some obvious oversight on my part.)

I am unable to get TextureWriter.filename to return the texture filename. Using this code:

# Display texture names
def display_tex_names
    tw = Sketchup.create_texture_writer
    load_textures(tw, Sketchup.active_model.entities)

    tex_index = 1
    while (tex_index <= tw.count)
        puts(tw.filename(tex_index))
        tex_index += 1        
    end

    tw.write_all("D;", false)

    Sketchup.active_model.commit_operation
end

outputs zero-length strings (not nil) to the Ruby console.

I know that the TextureWriter object was created successfully and that the texture files have valid names, because the files are written to the specified directory by TextureWriter.write_all. (By implication, the load_textures function is also working properly.)

What is supremely frustrating is that I was able to extract the filenames at one point in my code development, but I made a change and later encountered this problem.