Drivers

ALTO Image selects Imagick when it is available, then GD. Override selection for one request with using():

<?php

use Alto\Image\Driver\Gd\GdDriver;
use Alto\Image\Image;

$result = Image::open('photo.jpg')
    ->using(new GdDriver())
    ->fit(1200, 1200)
    ->webp()
    ->render();

Run vendor/bin/image doctor for the formats and delegates available on the current machine.

Built-in drivers

GD
Imagick

Feature	GD	Imagick
Raster input and output	Depends on compiled formats	Depends on compiled delegates
Animated input	First frame only	First frame only
Vector input	No	Rasterized at declared size
Metadata preservation	No	Supported where the delegate permits it
ICC profile conversion	No	Requires LCMS
Local operation behavior	Some effects are approximate	Arbitrary-angle rotation may be approximate
Batch execution	One decode per rendered source batch	One decode per rendered source batch

First-frame reads and vector rasterization are reported as Approximate, not silent exact support. Encoding effort and policy options can also vary by format and driver.

Capability levels

Drivers answer each concrete request with one of three levels:

Exact: performed as specified.
Approximate: performed with a documented degradation.
No: refused during negotiation.

When work is approximate, Result::$degradations explains the difference. Negotiation fails before decoding when no driver can perform the request.

Third-party drivers

Implement DriverInterface and verify it with the public conformance toolkit. See Writing a driver.

The driver contract reports a name, build capabilities, support for each operation, decode support for a source format, encode support for an output request, and processes one negotiated Plan. Capabilities exposes the exact read and write formats plus diagnostic notes for the installed build.

The supported testing namespace contains ArrayDriver, Corpus, DriverTestCase, and ImageAssertions. Extend DriverTestCase to apply the same behavioral contract used by the built-in drivers.