Why does AVAssetImageGenerator return a frame at a different time than you asked for, and how do you get the exact one?

Predict first

Which time comes back with each tolerance?

try await makeVideo(at: clip, frames: 30, fps: 30)
defer { try? FileManager.default.removeItem(at: clip) }

let generator = AVAssetImageGenerator(asset: AVURLAsset(url: clip))
let wanted = CMTime(seconds: 0.5, preferredTimescale: 600)

func grab(_ label: String) async throws {
    let result = try await generator.image(at: wanted)
    let grey = (result.image.dataProvider!.data! as Data)[0]
    print(label, "-> actualTime:", String(format: "%.3f", result.actualTime.seconds), "| frame grey level:", grey)
}
// Default tolerance is infinite: the generator may return any nearby frame it can decode cheaply.
generator.requestedTimeToleranceBefore = .positiveInfinity
generator.requestedTimeToleranceAfter = .positiveInfinity
let loose = try await generator.image(at: wanted)
print("infinite tolerance: asked 0.500, got", String(format: "%.3f", loose.actualTime.seconds))

generator.requestedTimeToleranceBefore = .zero
generator.requestedTimeToleranceAfter = .zero
try await grab("zero tolerance")

do { _ = try await generator.image(at: CMTime(seconds: 50, preferredTimescale: 600)) } catch { print("a time past the end throws") }

generator.maximumSize = CGSize(width: 16, height: 16)
let small = try await generator.image(at: wanted).image
print("maximumSize 16x16 gives", small.width, "x", small.height)

• 0.600 with infinite tolerance, 0.500 with zero, past the end throws, maximumSize gives 16 x 16
• both give exactly 0.500
• zero tolerance returns the nearest keyframe