Reset page geometry after Imagick rotation

[nlemoine]

Rotating an image with the Imagick driver leaves a page offset on every frame. You don't notice it on a still image. On an animated one that then gets encoded to AVIF, it breaks: the bottom-right of every frame comes out transparent.

Where it comes from: rotateImage() at a non-right angle grows the canvas and sets a negative page offset. A 480x480 frame rotated 45 degrees ends up with page 680x680-100-100. The single-frame AVIF encoder and the animated WebP encoder ignore that offset. The animated AVIF path does not, it composites every frame shifted up and to the left.

The real bug is in ImageMagick's animated AVIF writer, not in Intervention. But we can shield against it here, and the other modifiers already do this (see the fix below).

Repro with plain magick, no PHP involved:

magick in.gif -coalesce -background '#3b82f6' -rotate 45 bad.avif
magick 'bad.avif[0]' -alpha on -format '%[pixel:p{679,679}]' info:
# srgba(0,0,0,0)  -> transparent

magick in.gif -coalesce -background '#3b82f6' -rotate 45 +repage good.avif
magick 'good.avif[0]' -alpha on -format '%[pixel:p{678,678}]' info:
# srgba(59,130,246,1)  -> the background color, correct

Same thing through the library:

use Intervention\Image\ImageManager;
use Intervention\Image\Drivers\Imagick\Driver;
use Intervention\Image\Encoders\AvifEncoder;

$image = ImageManager::usingDriver(Driver::class)
    ->decodePath('in.gif')
    ->rotate(45, '#3b82f6');

$image->encode(new AvifEncoder());
// every frame is 680x680, bottom-right corner is transparent

Fix: reset each frame's page to +0+0 right after rotateImage(). TrimModifier and CoverModifier already do this after they change the canvas, so the rotate modifier is just catching up with them. GD and vips don't leave an offset after a rotate, so this also lines the drivers up.

One line in Drivers/Imagick/Modifiers/RotateModifier.php:

$frame->native()->setImagePage(0, 0, 0, 0);

Test: I added testRotateResetsFramePageOffset next to the existing rotate test. It rotates an animated fixture by 45 and checks every frame has offset 0. It fails before the change and passes after.

Corner pixel, same input, before and after:

driver	animated AVIF, bottom-right corner
imagick, before	transparent (srgba 0,0,0,0)
imagick, after	background color (srgba 59,130,246,1)
gd	fine (it flattens the animation to one frame)
vips	fine (no page offset)

Two things if you want to reproduce it:

It only shows with real photo-like frames. A flat color or a tiny synthetic gif compresses to trivial tiles and the corner stays fine, so use a real animated image.

You need an ImageMagick build with animated AVIF support. I saw it on 7.1.2-26, and it was also reported on a couple of earlier 7.1.2 builds. Older ImageMagick without animated AVIF is not affected.

[olivervogel]

Thank you.