-- spr_from_atlas() - draw to the screen a slice of a sprite atlas
-- param: s - sprite atlas to use (the number in the Picotron spritesheet)
-- param: idx - the 0-based index to render from the atlas
-- param: cols - number of columns in the sprite atlas
-- param: rows - number of rows in the sprite atlas
-- param: x - screen x-coordinate to draw top-left corner of the sprite
-- param: y - screen y-coordinate to draw top-left corner of the sprite
-- param: flip_x (optional) - flip the sprite horiztonally
-- param: flip_y (optional) - flip the sprite vertically
function spr_from_atlas(s, idx, cols, rows, x, y, flip_x, flip_y)
    -- assign default values to optional parameters if nil
    local fx = flip_x ~= nil and flip_x or false
    local fy = flip_y ~= nil and flip_y or false

    -- retrieve userdata about the sprite
    local spr_data = get_spr(s)
    local spr_w = spr_data:width()
    local spr_h = spr_data:height()

    -- calculate width and height of each atlas entry
    local w, h = (spr_w \ cols), (spr_h \ rows)

    -- determine where in the sprite's coordinate system to start referencing
    local spr_x = (idx % cols) * w
    local spr_y = (idx \ cols) * h

    -- sspr( sprite, sx, sy, sw, sh, dx, dy, dw, dh, flip_x, flip_y )
    sspr(s, spr_x, spr_y, w, h, x, y, w, h, fx, fy)
end