Perceptual precision causing differences to be missed in Bitrise CI due to failing CIContext.render() calls

[knellr]

Describe the bug

When running with perceptual precision < 1 we are observing that comparisons are picking up failures correctly locally, but are passing incorrectly on Bitrise CI.

To Reproduce

Run a snapshot test in Bitrise that is expected to fail with a perceptualPrecision < 1, for example as shown in the following project:

SnapshotIssue.zip

The issue appears to be with the following code (and similar) not updating the averagePixel var. Because it's initialised as zero, the tests pass - see

swift-snapshot-testing/Sources/SnapshotTesting/Snapshotting/UIImage.swift

Line 188 in e408bf9

var averagePixel: Float = 0

  var averagePixel: Float = 0
  let context = CIContext(options: [.workingColorSpace: NSNull(), .outputColorSpace: NSNull()])
  context.render(
    thresholdOutputImage.applyingFilter("CIAreaAverage", parameters: [kCIInputExtentKey: new.extent]),
    toBitmap: &averagePixel,
    rowBytes: MemoryLayout<Float>.size,
    bounds: CGRect(x: 0, y: 0, width: 1, height: 1),
    format: .Rf,
    colorSpace: nil
  )

I'm not familiar with this call and this may be a Bitrise specific issue as they're running in VMs (I have raised a ticket with Bitrise) but I think it should be possible to make the tests fail if this scenario is hit.

Expected behavior

Test should fail

Environment

• swift-snapshot-testing version 1.11.0
• Xcode 14.2
• Swift 5.7
• OS: iOS 16

Additional context

xctestresult for Incorrectly passing run (Bitrise):

Test-SnapshotIssue.xcresult.zip

Logs

xcodebuild_test.log

[nshelleyacorns]

I'm pretty sure this is because perceptual precision requires metal and VMs don't support metal. I ran into the same issue on CircleCI. Unfortunately, without metal support all tests using perceptual precision would pass, but that has been fixed in #702. (But it still hasn't been merged and no word from the maintainers... 😞)

[knellr]

Ah yes, it seems likely the same thing. Interestingly Bitrise did have Metal support, but apparently it was turned off last March and I guess maybe not turned back on again https://bitrise.io/blog/post/run-faster-ios-ui-tests-via-metal-support

[knellr]

Ran the branch in #702 and got the failure I needed all along

Assertions: Assertion Failure at SnapshotIssueTests.swift:18: failed - Snapshot does not match reference.

@−
"file:///Users/vagrant/git/SnapshotIssueTests/__Snapshots__/SnapshotIssueTests/testSnapshot.1.png"
@+
"file:///Users/vagrant/Library/Developer/XCTestDevices/39F9E0E8-C99C-4BD6-91B7-14021B8BBF34/data/Containers/Data/Application/1F6B607B-DD35-4997-8408-9496F3F51C8F/tmp/SnapshotIssueTests/testSnapshot.1.png"

To configure output for a custom diff tool, like Kaleidoscope:

    SnapshotTesting.diffTool = "ksdiff"

Failed to compare snapshots. Metal is required for perceptuallyCompare, but not available on this machine.
  File: SnapshotIssueTests.swift:18

[knellr]

Bitrise have confirmed that they currently only support Metal in their M1 environments.

[tdrhq]

I think the issue here is recording screenshots in a different environment than where it's being verified from. Ideally you'd record screenshots in CI.

[knellr]

Closing this as this is no longer an issue in Bitrise, though #666 or #702 may help in equivalent environments