FCP 11.0 - 11.2 Magnetic Mask vertical line artifacts starting with Apple Silicon macOS Tahoe 26.0 #496
Description
Apple Feedback Assistant ID: FB20670391
DESCRIBE THE BUG:
Starting with Apple Silicon macOS Tahoe 26.0, FCP versions 11.0 to 11.2 exhibit intermittent vertical line artifacts if using Magnetic Mask. It is independent of codec and resolution. Does not happen on Intel.
The artifacts are typically seen as periodic, fluctuating single-frame instances of thin, sparse pixel-like vertical columns. If using 3840 x 2160 resolution, there are three columns separated horizontally by 1024 pixels. While appearing pixel-like in size, the objects composing each vertical column are all 6 pixels wide and normally 1 or 2 pixels tall, sparsely populated such that about 25% of the pixels in a 6-pixel-wide vertical column are affected. See attached screen captures.
FCP 11.0 was the first version with Magnetic Mask, so this obviously does not happen in prior FCP versions. It happens on 11.0 through 11.2 on Tahoe 26.0 and 26.0.1.
The problem visually appears as intermittent, flickering, single-frame thin vertical lines. However, overall reproducibility is good, approaching 100% for the test case. It can be harder to visually see if using only the Magnetic Mask effect by itself with overlay enabled, so it’s best to use Color Wheels with the Magnetic Mask (available as an additional option to shape and color masks). Then turn global brightness down to black, and the artifacts easily stand out.
The artifacts can appear immediately upon drawing the mask, even before analyzing. However, the combination of analyzing and using the mask within a color effect makes it easier to see. It happens before rendering to cache, but persists in cache and export.
TO REPRODUCE:
- Import 3840x2160/23.98 H.264 test clip. Any codec or resolution will work, but UHD 4k seems to show it well. At 1080p it may only show a single vertical line.
- Add to auto timeline
- Apply Color Wheels effect
- Within Color Wheels effect, click mask button and add Magnetic Mask
- Effect defaults to eyedropper + cursor. Simply click on image several times to fill screen with mask.
- Click “Analyze” OSC
- Click “Done” OSC
- Pull down global brightness to dark
- Play clip. It should manifest 100% of the time for the above case.
EXPECTED BEHAVIOUR:
One to three intermittent single-frame artifacts consisting of thin vertical lines (ie columns). On 3840 x 2160 frames, the three columns are separated by 1024 horizontal pixels.
SCREENSHOTS:
- Screen cap: FCP_11-2_MacOS_Tahoe_Magnetic_Mask_defect_full_frame.jpg
- Screen cap: FCP_11-2_MacOS_Tahoe_Magnetic_Mask_defect_close_up.jpg
SPECS:
MACHINES AND VERSIONS TESTED:
- M1 Ultra Mac Studio 128GB, FCP 11.0 - 11.2, macOS Tahoe 26.0.1
- M1 Max Mac Studio FCP 11.0 - 11.2, macOS Sequoia 15.7.1 — Does not happen.
- 2019 i9 MacBook Pro 16, FCP 11.0 - 11.2, macOS Sequoia 26.0.1 - problem does not happen on Intel.
