Projects: Motion Masks

by David Thomas on

Bitmap compositing driven by RLE-compressed alpha masks.

Keywords: RLE alpha animation bitmap blend buffer clip clipping compact composite compressed framebuffer mask motion opacity stencil transition transparency wipe

The Idea

Let’s say you have two static images:

Two static images.

and you want to create an animated transition between them. For example, a fade:


or a wipe:


To describe each frame of the animation you could use a third image to specify at every pixel which source image to use. The “lowest” pixel value would mean select the corresponding pixel from the first image, the “highest” pixel value would mean select from the second image. Other pixel values would output a blend of the two respective input pixels.

Using green to represent “high” and black for “low”, the masks for our fade example would be:

Fade mask.

and our wipe:

Wipe mask.

Of course, this is will be wasteful: the animation’s frames are likely to be ‘sparse’, containing long runs of identical intensity but few ‘edge’ pixels.

The sparseness of individual scanlines means that they are ideal for adaptive RLE compression. This saves space and when rendering would save waiting over and over for RAM fetches. The compression, in effect, makes use of wasted CPU cycles to provide faster RAM access.

Additionally, many entire scanlines will be identical, especially when considering all frames in the animation. We can therefore factor out identical scanlines by considering a frame to be compressed scanlines indirected through a table of offsets.

Now with our offsets and compressed scanlines we can render the animation by stepping through the offsets and plotting pixels as directed by the compressed scanline data.


The compression method used is RLE. This is simple enough to not require any heavy grunt work while plotting, but small enough to allow memory bandwidth requirements to be reduced.

Compression Method

  1. All of the input bitmaps are loaded.
  2. Each scanline is numbered and hashed then recorded in an array.
  3. The array is sorted into (hash, bitmap index, scanline index) order.
  4. Duplicates scanlines are discarded.
  5. Non-duplicates are encoded and packed into a data buffer, offsets retained.


This is just like a regular bitmap plot except that each row decodes a sequence of commands. The commands specify whether to plot, or blend between the corresponding two source image pixels and plot the result.

Two source images are available concurrently. The copy command can select either, the blend commands implicitly use both.

Other commands exist, such as one which selects the respective source images for copies and blends.

Bitmaps & Screens

The Motion Mask code provides its own abstractions for bitmaps, screens and pixel formats, rather than operating on OS-specific types:

There is also a bitmap_set_t which is just like a bitmap_t but provides for multiple base pointers, allowing a series of images with identical dimensions and depth to be specified.

Pixel Formats

Pixel formats are specified with a pixelfmt_t. This enumeration has many pixels formats but only two are implemented so far: rgbx8888 and xbgr8888.

Pixel Handlers

The copying and blending of specific pixel formats are coded in a span_t.

Source Code Organisation

Building the Test App

The core is generic, but the test app only exists for OS X presently.


Current Status

There is an OS X Cocoa test app (“MotionMaskTest”) in platform/macos which loads a PNG from a (presently hard coded) location. This is packed into a Motion Mask and written out to disc. It’s then loaded back in and some JPEGs are loaded and displayed through the Motion Mask. The mouse can be moved around the window to draw at different offsets.

File Format





Binary Encoding

The count of leading zeros of the initial byte is the unique identifier of the operation.


Command Binary format Description
Copy 1SLLLLLL Copy from source S 1..64 pixels.
Blend const 01LLLLLL AAAAAAAA Alpha blend 1..64 pixels with constant alpha.
Blend array 001LLLLL AAAAAAAA[len] Alpha blend 1..32 pixels with variable alpha.
Long copy 0001SLLL LLLLLLLL Copy from source S 65..2112 pixels.
Long blend const 00001LLL LLLLLLLL AAAAAAAA Alpha blend 65..2112 pixels with constant alpha.
Long blend array 000001LL LLLLLLLL AAAAAAAA[len] Alpha blend 33..1056 pixels with variable alpha.
Set source 0000001x TTTTSSSS Set source images 0 and 1 to S and T. (Source zero is the screen, source N is input image N-1).
EOL 00000001 End of line.

To Do


Other Projects with Similar Features


David Thomas


Copyright © David Thomas, 2012. All Rights Reserved.

[Licensing will be sorted out once the damn thing works].