ordinal position of images in catalog (which is the same as the filmstrip order?)

My function for getting photos in filmstrip, for example purposes (requires Elare Plugin Framework to function):

--- Get photos visible in filmstrip (as filtered, as stacked, ... ).
--
--  @usage without disturbing users's present selection.
--
--  @param selectedPhoto (LrPhoto, default=nil) catalog:getTargetPhoto() - in case already available in calling context.
--  @param selectedPhotos (array of LrPhoto, default=nil) catalog:getTargetPhotos() - ditto.
--  @param metadataCache (LrMetadata::Cache, optional) metadata cache.
--  @param includeIfBuried (boolean, default=false) visible only? or those underneath in collapsed stacks too.
--
--  @return visiblePhotos (array) never nil, but may be empty (e.g. virgin catalog)
--
function Catalog:getVisiblePhotos( params )
    -- works in Lr3+4+5/win7; ###1 test on Mac.
    params = params or {}
    local selectedPhoto = params.selectedPhoto or catalog:getTargetPhoto()
    local selectedPhotos = params.selectedPhotos or catalog:getTargetPhotos()
    local visiblePhotos
    if selectedPhoto then
        catalog:setSelectedPhotos( selectedPhoto, {} )
        local allVisiblePhotos = catalog:getMultipleSelectedOrAllPhotos()
        catalog:setSelectedPhotos( selectedPhoto, selectedPhotos )
        --Debug.pause( #allVisiblePhotos, #selectedPhotos, #selectedPhotos == #catalog:getTargetPhotos() ) 
        visiblePhotos = allVisiblePhotos
    else
        visiblePhotos = selectedPhotos
    end
    if not params.includeIfBuried or #visiblePhotos == 0 then
        return visiblePhotos
    end
    -- fall-through => include underlings.
    local cache = params.metadataCache or lrMeta:createCache{ photos=visiblePhotos, rawIds={ 'isInStackInFolder', 'stackInFolderIsCollapsed', 'stackInFolderMembers', } }
    local buriedPhotos = {}
    for i, photo in ipairs( visiblePhotos ) do
        repeat                
            if not cache:getRawMetadata( photo, 'isInStackInFolder' ) then
                break
            end
            -- photo is in stack
            if not cache:getRawMetadata( photo, 'stackInFolderIsCollapsed' ) then
                break
            end
            -- stack is collapsed, therefore it is top of stack (actually, that is only true if source is folder - viewing collection, one could view underlings of collapsed folder stacks).
            local stack = cache:getRawMetadata( photo, 'stackInFolderMembers' )
            --assert( stack[1] == photo, "tos?" ) - invalid assertion (see comment above).
            for j, stackedPhoto in ipairs(stack) do
                if photo ~= stackedPhoto then
                    buriedPhotos[#buriedPhotos + 1] = stackedPhoto
                end
            end
        until true
    end
    tab:appendArray( visiblePhotos, buriedPhotos ) -- returns promptly if no buried photos.
    return visiblePhotos
end