There is currently no OSD indicator for brightness when controlling backlight for screenpad devices. This may limit the practicality of ACPI patches that implement more than a handful of brightness steps. Refer to SSDT-SPLC.dsl#L42-L63 for an implementation example.
There is already a function for extracting and calling the same OSD method in AsusSMC (AsusSMCDaemon/OSD.h), though perhaps a clearer reference comes from ExternalDisplayBrightness for handling multiple displays:
// @fnesveda/ExternalDisplayBrightness/src/ExternalDisplayBrightness/BrightnessManager.swift#L330-L349
330 | // shows the brightness HUD with the correct brightness indicator on the correct display
331 | // unfortunately Apple doesn't provide a public API for this, so we have to manually extract the function from the OSD framework
332 | private static var showBrightnessHUD: ((CGDirectDisplayID, Int, Int) -> Void)? {
333 | if let osdLoaded = Bundle(path: "/System/Library/PrivateFrameworks/OSD.framework")?.load(), osdLoaded {
334 | if let sharedOSDManager = NSClassFromString("OSDManager")?.value(forKeyPath: "sharedManager") as AnyObject? {
335 | let showImageSelector = Selector(("showImage:onDisplayID:priority:msecUntilFade:filledChiclets:totalChiclets:locked:"))
336 | if sharedOSDManager.responds(to: showImageSelector) {
337 | let showImageImplementation = sharedOSDManager.method(for: showImageSelector)
338 |
339 | typealias ShowImageFunctionType = @convention(c) (AnyObject, Selector, UInt32, UInt32, UInt32, UInt32, UInt32, UInt32, Bool) -> Void
340 | let showImage = unsafeBitCast(showImageImplementation, to: ShowImageFunctionType.self)
341 |
342 | return {(onDisplayID: CGDirectDisplayID, filledChiclets: Int, totalChiclets: Int) -> Void in
343 | showImage(sharedOSDManager, showImageSelector, 1, onDisplayID, 0x1f3, 1000, UInt32(filledChiclets), UInt32(totalChiclets), false)
344 | }
345 | }
346 | }
347 | }
348 | return nil
349 | }