Editing Image Formats (section)

=== Tall patches ===
''Tall patches'' are a backwards-compatible hack to allow Doom pictures larger than 256 pixels in height. Since Doom pictures can theoretically be up to 65536x65536 pixels large, yet posts can only be up to 254 pixels long (with the byte value <code>255</code> serving as a terminating byte per-post), vanilla Doom pictures can practically only be 65536x508 pixels large. Tall patches hack around this limit by placing posts beyond <code>y=255</code> relatively to one another, theoretically allowing for an infinite vertical size.

In a Doom engine supporting tall patches, whenever a post's top delta is less than it's predecessor's top delta, an overflow is detected and both values are added together for the new top delta to allow for the tall patch. This makes top offsets beyond 254 effectively relative to one another, rather than absolute as before.

In a Doom engine ''not'' supporting tall patches, the posts in question will simply wrap and draw over the existing sprite space.

Many tools like [https://slade.mancubus.net/ SLADE] and [https://github.com/liquidunderground/pk3make PK3Make] will also automatically insert zero-length pseudo patches at top delta 254 during image conversion to guarantee a reliable detection of tall patches.
[[Category:Technology]]