How GIMP Uses Copy-on-Write for Layer Duplication

This article explains how the GNU Image Manipulation Program (GIMP) employs a copy-on-write (CoW) architecture through its core image processing engine, GEGL, to duplicate layers without duplicating physical memory usage. By breaking images into discrete tiles, managing tile pointers through reference counting, and only allocating new memory when pixels are actively modified, GIMP allows users to work with multiple heavy layers while keeping memory consumption to a minimum.

The Role of GEGL and Tile-Based Storage

At the center of GIMP’s memory management is GEGL (Generic Graphics Library). Rather than storing an entire image layer as a single, contiguous block of pixel data in RAM, GEGL abstracts the pixel data into a structure known as GeglBuffer.

A GeglBuffer divides an image into a uniform, two-dimensional grid of smaller rectangular blocks called tiles (typically 64×64 or 128×128 pixels). Each tile contains its own pixel data, color format metadata, and memory management parameters. This tiled architecture is the foundational requirement for GIMP's copy-on-write functionality because it allows operations to target localized segments of an image rather than the entire canvas.

Reference Counting Instead of Data Duplication

When you duplicate a layer in GIMP, the application does not make an immediate, deep copy of the underlying pixel data in physical RAM. Instead, it performs a lightweight, shallow copy:

  1. New Buffer Creation: GIMP instantiates a new GeglBuffer to represent the duplicated layer.
  2. Tile Pointer Mapping: The new buffer creates an index of tile coordinates that points to the exact same physical memory addresses (the existing GeglTile instances) used by the original layer.
  3. Reference Count Increment: Each shared tile contains a reference counter. When the duplicate layer maps to an existing tile, that tile's reference count is incremented by one.

Because the process only clones the pointer array and metadata, duplicating a 4K or 8K layer occurs almost instantaneously and consumes only a negligible amount of RAM.

The Copy-on-Write (CoW) Trigger

The shared state persists until one of the layers is modified by a tool, brush stroke, or filter. When a write operation is initiated, GIMP enforces the copy-on-write protocol:

  1. Tile Interception: The graphics engine determines the precise bounding box of the edit and identifies which specific tiles fall within the modified area.
  2. Reference Check: Before modifying the pixel data of a target tile, GEGL inspects the tile's reference count. If the reference count is 1, the tile is unique to that layer, and GEGL writes directly to the existing memory space.
  3. Tile Cloning: If the reference count is greater than 1, the tile is actively shared. GEGL clones only that specific tile, allocating new physical RAM for the copy.
  4. Pointer Reassignment: The modified layer updates its tile grid to point to the newly allocated tile, while the original layer retains the pointer to the original tile.
  5. Reference Count Decrement: The reference count on the original tile is decremented by one.

Memory Benefits and Swap Interaction

By operating at the tile level, GIMP ensures that if a user duplicates a 500 MB layer and paints a small stroke affecting only four tiles, GIMP only allocates memory for those four tiles (a few kilobytes). The remaining unedited tiles remain shared between both layers.

Furthermore, this tile-based CoW system interfaces directly with GIMP's tile cache and disk-swapping backend. When physical RAM limits are approached, GEGL can page unmodified, shared tiles to disk once, rather than paging duplicate copies for every layer that references them, further stabilizing system performance under heavy workloads.