NexusTK EPF Transparency Mask Format - Standard or Custom?

[DizzyThermal]

Hello!

I'm trying to understand a binary format of an image transparency mask for NexusTK - a 2D MMORPG from 1996.

(Disclaimer: I received help back in the XeNTaX days, and no longer have a way to contact who helped, so my understanding of this format is not great - that's why I'm asking here ).

To build my transparency masks, I use the following code - it's GDScript so the code is python-ish:

var STENCIL_MASK := 0x80
var mask_image := Image.create(width, height, false, Image.FORMAT_RGBA8)

for i in range(height):
    var total_pixels := 0

    while true:
        var pixel_count := read_u8()

        if pixel_count == 0x0:
            break  # End of mask

        var should_draw := false
        if pixel_count > STENCIL_MASK:
            should_draw = true

        if should_draw:
            # Not sure why this works
            pixel_count = pixel_count ^ STENCIL_MASK

        for j in range(pixel_count):
            var pixel_color := Color.TRANSPARENT
            if should_draw:
                pixel_color = Color.BLACK
            mask_image.set_pixel(total_pixels, i, pixel_color)
            total_pixels += 1

    if total_pixels < width:
        for j in range(width - total_pixels):
            mask_image.set_pixel(total_pixels, i, Color.TRANSPARENT)

An example mask and image are:

The mask is used to blit out the background pixels that aren't suppose to be rendered (pink pixels above).

Is this image mask format common or custom (like I'm not sure why 0x80 is a special value)?

Is there a better way I can build this transparency mask, instead of pixel by pixel?

Thanks!

-DizzyThermal