zenlayout

zenlayout is a pure-geometry image layout engine for computing resize dimensions, crop regions, and canvas placement.

no_std compatible, #![forbid(unsafe_code)].

[dependencies]
zenlayout = "0.2"
# With RIAPI query string parsing:
zenlayout = { version = "0.2", features = ["riapi"] }

What it does

Given source dimensions and a set of commands (orient, crop, region, constrain, pad), zenlayout computes every dimension, crop rect, and placement offset needed to produce the output. It handles EXIF orientation, aspect-ratio-aware scaling, codec alignment (JPEG MCU boundaries), and gain map / secondary plane spatial locking.

What it doesn't do: touch pixels. That's your resize engine's job.

Quick start

use zenlayout::{Pipeline, DecoderOffer, OutputLimits, Subsampling};

let (ideal, request) = Pipeline::new(4000, 3000)
    .auto_orient(6)            // EXIF orientation 6 = 90 CW
    .fit(800, 600)             // fit within 800x600
    .output_limits(OutputLimits {
        align: Some(Subsampling::S420.mcu_align()),
        ..Default::default()
    })
    .plan()
    .unwrap();

// Pass `request` to your decoder, get back what it actually did
let offer = DecoderOffer::full_decode(4000, 3000);
let plan = ideal.finalize(&request, &offer);

// plan.resize_to, plan.canvas, plan.remaining_orientation, etc.
// contain everything the resize engine needs

RIAPI query strings

With the riapi feature, parse URL query strings directly:

use zenlayout::riapi;

let result = riapi::parse("w=800&h=600&mode=crop&scale=both");
let pipeline = result.instructions
    .to_pipeline(4000, 3000, None)
    .expect("valid pipeline");

let (ideal, _request) = pipeline.plan().expect("valid layout");
assert_eq!(ideal.layout.resize_to.width, 800);
assert_eq!(ideal.layout.resize_to.height, 600);

Supported parameters: w/h, maxwidth/maxheight, mode, scale, crop, anchor, zoom/dpr, srotate/rotate, flip/sflip, autorotate, bgcolor, c.gravity, c.focus, c.zoom, c.finalmode. Non-layout keys (format, quality, effects) are preserved in Instructions::extras() for downstream consumers.

Smart crop parameters

These parameters control content-aware crop positioning. They require a crop-producing mode (mode=crop) with target dimensions (w/h).

Example: ?w=400&h=400&mode=crop&c.focus=20,30,80,90 crops to a square while keeping the region from 20-80% width, 30-90% height visible.

Parsed into Instructions::c_focus (CFocus enum), c_zoom (Option<bool>), and c_finalmode (Option<String>). Back-compatible with ImageResizer's CropAround plugin syntax.

Processing pipeline

The Pipeline builder processes operations in a fixed order, regardless of the order setters are called. Last-setter-wins: calling the same category twice replaces the previous value (standard builder pattern). Orientation is the exception — it always composes algebraically.

Pipeline processing order

1. ORIENT -- All orientation commands (auto_orient, rotate, flip) compose
   into a single source transform via D4 group algebra. This happens
   regardless of where they appear -- there is no "post-resize flip."
   Source dimensions transform to post-orientation space.

     .auto_orient(6).rotate_90() = Rotate90 . Rotate90 = Rotate180
     .fit(800, 600).flip_h()     = flip source, then fit (not "fit then flip")

2. REGION or CROP -- Define the effective source. Crop and region share a
   single slot; setting either replaces the other.
   - Crop: select a rectangle within the source (origin + size)
   - Region: viewport into infinite canvas (edge coords; can crop, pad, or both)
   Crop converts to Region internally.

3. CONSTRAIN -- Resize the effective source to target dimensions. The 9
   constraint modes control aspect ratio handling.
   - Fit/FitCrop/FitPad/Distort will upscale small images
   - Within/WithinCrop/WithinPad will not
   - PadWithin never upscales but always pads to target canvas
   - Single-axis constraints derive the missing dimension from aspect ratio

4. PAD -- Add explicit padding around the constrained result. Additive on
   canvas dimensions. Padding does NOT collapse -- pad_uniform(10, color)
   always adds exactly 10px on each side regardless of other commands.

5. LIMITS -- Safety limits applied to the final canvas:
   a. max -- proportional downscale if canvas exceeds max (security cap)
   b. min -- proportional upscale if canvas below min (quality floor)
   c. align -- snap to codec multiples (may slightly exceed max/drop below min)
   Max always wins over min.

Sequential mode (compute_layout_sequential()): same operations, but commands execute in order. Orient still fuses into a source transform. Multiple crop/region compose (each refines the previous). Last constrain wins. Post-constrain crop/pad adjusts the output canvas.

Two-phase layout

Layout computation splits into two phases to support decoder negotiation (JPEG prescaling, partial decode, hardware orientation):

    Commands + Source
          |
          v
    +--------------+     +--------------+
    |compute_layout|---->|DecoderRequest|----> Decoder
    +--------------+     +--------------+        |
          |                                      |
          v                                      v
    +-----------+       +-------------+    +------------+
    |IdealLayout|------>|  finalize() |<---|DecoderOffer|
    +-----------+       +-------------+    +------------+
                              |
                              v
                        +----------+
                        |LayoutPlan| -- final operations
                        +----------+

Phase 1 (Pipeline::plan() or compute_layout()) computes the ideal layout assuming a full decode. It returns an IdealLayout (what the output should look like) and a DecoderRequest (hints for the decoder — crop region, target size, orientation).

Phase 2 (IdealLayout::finalize()) takes a DecoderOffer describing what the decoder actually did (maybe it prescaled to 1/8, applied orientation, or cropped to MCU boundaries). It compensates for the difference and returns a LayoutPlan with the remaining work: what to trim, resize, orient, and place on the canvas.

If your decoder doesn't support any of that, pass DecoderOffer::full_decode(w, h).

Constraint modes

Nine modes control how source dimensions map to target dimensions:

WithinPad vs PadWithin: when the source is smaller than the target, WithinPad returns the image at its original size (identity — no canvas expansion). PadWithin always returns the target canvas dimensions with the image centered on it.

    Source 4:3, Target 1:1 (square):

    Fit           Within         FitCrop       FitPad
    +---+         +---+          +---+         +-----+
    |   |         |   |          | # |         |     |
    |   |         |   |          | # |         | ### |
    |   |         |   |(smaller) | # |         |     |
    +---+         +---+          +---+         +-----+
    exact size    <= source      fills+crops    fits+pads

Single-axis constraints are supported: Constraint::width_only() and Constraint::height_only() derive the other dimension from the source aspect ratio.

Orientation

Orientation models the D4 dihedral group — 4 rotations x 2 flip states = 8 elements, matching EXIF orientations 1-8.

Orientations compose algebraically and are verified against the D4 Cayley table:

use zenlayout::Orientation;

let exif6 = Orientation::from_exif(6).unwrap(); // 90 CW
let combined = exif6.compose(Orientation::FlipH);
assert_eq!(combined, Orientation::Transpose);   // EXIF 5

// Inverse undoes:
assert_eq!(exif6.compose(exif6.inverse()), Orientation::Identity);

All orientation commands fuse into a single source transform, regardless of where they appear in the pipeline. There is no "post-resize flip" — orientation is always applied to the source. In sequential mode, if an axis-swapping orientation (Rotate90/270, Transpose, Transverse) appears after a constraint, the constraint's target dimensions are swapped to compensate, producing correct output geometry.

Region

Region defines a viewport rectangle in source coordinates. It unifies crop and pad into a single concept:

• Viewport smaller than source = crop
• Viewport extending beyond source = pad (filled with color)
• Viewport entirely outside source = blank canvas

Coordinates use edge positions (left, top, right, bottom), not origin + size. Region::crop(10, 10, 90, 90) selects an 80x80 area. This differs from SourceCrop::pixels(10, 10, 80, 80) which uses origin + size for the same region.

Each edge is a RegionCoord: a percentage of source dimension plus a pixel offset. This allows expressions like "10% from the left edge" or "50 pixels past the right edge".

use zenlayout::{Pipeline, Region, RegionCoord, CanvasColor};

// 50px padding on all sides
let (ideal, _) = Pipeline::new(800, 600)
    .region(Region::padded(50, CanvasColor::white()))
    .plan()
    .unwrap();
// Canvas: 900x700, source at (50, 50)

// Mixed crop+pad: extend left, crop right
let (ideal, _) = Pipeline::new(800, 600)
    .region_viewport(-50, 0, 600, 600, CanvasColor::black())
    .plan()
    .unwrap();
// Canvas: 650x600, 600x600 of source at (50, 0)

// Percentage-based crop: 10% from each edge
let reg = Region {
    left: RegionCoord::pct(0.1),
    top: RegionCoord::pct(0.1),
    right: RegionCoord::pct(0.9),
    bottom: RegionCoord::pct(0.9),
    color: CanvasColor::Transparent,
};

SourceCrop converts to Region internally via to_region(). Region and Crop share a single slot — setting either replaces the other.

When a Region is combined with a constraint, the constraint operates on the overlap between the viewport and the source. The viewport's padding areas scale proportionally.

Sequential mode

For scripting use cases where command order matters, use compute_layout_sequential() with a Command slice:

use zenlayout::{compute_layout_sequential, Command, SourceCrop};

let commands = [
    Command::Crop(SourceCrop::pixels(100, 100, 600, 400)),
    Command::Crop(SourceCrop::pixels(50, 50, 500, 300)),  // refines the first crop
];
let (ideal, _) = compute_layout_sequential(&commands, 800, 600, None).unwrap();

Sequential mode differences from fixed mode:

• Orient: still fuses into a single source transform regardless of position
• Crop/Region: compose sequentially (second crop refines the first)
• Constrain: last one wins
• Post-constrain crop/pad: adjusts the output canvas, not the source
• Limits: applied once at the end (same as fixed)

Both modes produce a single Layout — one crop, one resize, one canvas. "Sequential" refers to the command evaluation order, not multi-pass pixel processing. Requires alloc feature.

Secondary planes

For gain maps, depth maps, or alpha planes that share spatial extent with the primary image but live at a different resolution:

use zenlayout::{Pipeline, DecoderOffer, Size};

// SDR: 4000x3000, gain map: 1000x750 (1/4 scale)
let (sdr_ideal, sdr_req) = Pipeline::new(4000, 3000)
    .auto_orient(6)
    .crop_pixels(100, 100, 2000, 2000)
    .fit(800, 800)
    .plan()
    .unwrap();

// Derive gain map plan -- automatically maintains the source ratio
let (gm_ideal, gm_req) = sdr_ideal.derive_secondary(
    Size::new(4000, 3000),  // primary source
    Size::new(1000, 750),   // gain map source
    None,                   // auto: 1/4 of SDR output
);

// Each decoder independently handles its capabilities
let sdr_plan = sdr_ideal.finalize(&sdr_req, &DecoderOffer::full_decode(4000, 3000));
let gm_plan = gm_ideal.finalize(&gm_req, &DecoderOffer::full_decode(1000, 750));

// Both plans produce spatially-locked results
assert_eq!(sdr_plan.remaining_orientation, gm_plan.remaining_orientation);

Source crop coordinates are scaled from primary to secondary space with round-outward logic (origin floors, extent ceils) to ensure full spatial coverage.

Codec layout

CodecLayout computes per-plane geometry for YCbCr encoders:

use zenlayout::{CodecLayout, Subsampling, Size};

let codec = CodecLayout::new(Size::new(800, 608), Subsampling::S420);

// Luma plane
assert_eq!(codec.luma.extended, Size::new(800, 608));
assert_eq!(codec.luma.blocks_w, 100); // 800 / 8

// Chroma plane (half resolution for 4:2:0)
assert_eq!(codec.chroma.extended, Size::new(400, 304));

// MCU grid
assert_eq!(codec.mcu_size, Size::new(16, 16));
assert_eq!(codec.mcu_cols, 50);

// Feed rows in chunks of this size to the encoder
assert_eq!(codec.luma_rows_per_mcu, 16);

Feature flags

The core API (Pipeline, Constraint::compute(), compute_layout()) works with zero features — no_std, no heap. Pipeline::plan() makes zero heap allocations.

Error handling

LayoutError is returned from Constraint::compute(), Pipeline::plan(), and Instructions::to_pipeline():

NaN/Inf values are rejected at all API boundaries — in the RIAPI parser, at Constraint::compute() entry, and at Instructions::to_pipeline() entry.

Limitations

• Only integer coordinates (no subpixel positioning)
• Sequential mode requires alloc feature
• smart-crop feature is experimental, API unstable
• No pixel operations — geometry only

Image tech I maintain

_{* as of 2026}

General Rust awesomeness

archmage · magetypes · enough · whereat · zenbench · cargo-copter

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License

Dual-licensed: AGPL-3.0 or commercial.

I've maintained and developed open-source image server software — and the 40+ library ecosystem it depends on — full-time since 2011. Fifteen years of continual maintenance, backwards compatibility, support, and the (very rare) security patch. That kind of stability requires sustainable funding, and dual-licensing is how we make it work without venture capital or rug-pulls. Support sustainable and secure software; swap patch tuesday for patch leap-year.

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