Four Stages, One Still

A filter chain is easy to hide inside one UIImage. This bench refuses to hide it.

One Harbor still. Four columns. ORIGINAL is the control — no CIFilter. Everything to its right takes the previous CIImage as input. Sepia intensity is the live knob.

Rest — sepia 0.15

-mode rest

ORIGINAL keeps the dusk gradient and yellow beacon. NOIR is grayscale. SEPIA at 0.15 is only a hint of warmth on that grayscale. BLOOM softens the highlights of that sepia output.

Mid — sepia 0.55

-mode mid

ORIGINAL and NOIR do not move — they do not read the sepia knob. SEPIA and BLOOM both warm because bloom is fed by sepia’s output, not by the original.

Hot — sepia 1.00

-mode hot

noir.inputImage = original
sepia.inputImage = noir.outputImage
sepia.intensity = Float(sepiaAmount)
bloom.inputImage = sepia.outputImage

Mini-exercise

Swap the chain so sepia runs on the original and noir runs after. Capture -mode hot and compare the SEPIA column to today’s brown-on-gray.

Challenges

1. Add a fifth stage: CIColorControls contrast on bloom output.
2. Show a parallel strip that applies each filter to ORIGINAL independently (not cumulative) and label the difference.
3. Drive sepia from a TimelineView instead of DispatchQueue asyncAfter.
4. Export the BLOOM stage to the photo library via CIContext.writePNGRepresentation.
5. Unit-test that ORIGINAL’s rendered pixels match the source when sepia is 1.0.

Key Points

• Observed SEPIA pins: 0.15, 0.55, 1.00.
• ORIGINAL never tints — that is the control.
• Each stage’s inputImage is the previous stage’s outputImage.
• Bloom crops back to the source extent after radius expansion.
• Live intensity proves which columns depend on the knob.
• Filter order is a product decision, not a detail.

Next up: Metal shaders in SwiftUI — colorEffect / distortionEffect / layerEffect on the same view, one uniform.