Over-bright white edges around white artwork when decoding 8-bit Gray+Alpha PNG on real device (SDWebImage 5.21.7, iOS 26.6.2) #3888

What happened?

On a real iPhone, an image loaded with SDWebImage shows a ~1–2 px over-bright white ring on the anti-aliased edge of white artwork. The ring looks like HDR "super white": it is brighter than the pure white pixels of the same image.

The exact same PNG file loaded with UIImage(contentsOfFile:) on the same device looks correct.

Only white artwork shows the ring. Black / dark artwork with the same PNG encoding and the same kind of semi-transparent edges does not show it.

Source images that trigger the bug

Both are 450×450, 8-bit PNG, Gray + Alpha (color type 4), Generic Gray Gamma 2.2 profile. The anti-aliased edge is made of semi-transparent white pixels (for example gray=255, alpha≈7).

How it looks on a real device

Photos of the actual screen. The circle sticker shows a thin bright ring around its edge when loaded with SDWebImage.

How to reproduce

1. Take fragile_label.png above (host it on a URL, or save it locally).
2. On a real iPhone (not Simulator), show it in a UIImageView on a light background (we used #F7F7F9), and decode it two ways:

// B — wrong rendering: bright white ring around the white circle edge
imageView.sd_setImage(with: url)

// A — correct rendering: the same file, without SDWebImage
imageView.image = UIImage(contentsOfFile: localPath)

3. Compare the edge of the white circle:
   • With SDWebImage: the edge is brighter than the pure white content of the image.
   • With UIImage(contentsOfFile:): the edge looks normal.

A 204×204 downscaled version of the same artwork (same Gray+Alpha encoding) also reproduces the ring.

What we measured (informational only)

• A screenshot of the wrong rendering contains a gain map. Measured in extended linear Display P3, the ring luminance is about 4.5× SDR white (page background ≈ 1.0). So the ring is really brighter than white — not a visual illusion.
• Decoded-image properties (real-device log):

A (UIImage direct): 450x450 bpc=8 bpp=16 alphaInfo=.last (raw 3)             colorSpace=Generic Gray Gamma 2.2
B (SDWebImage):     450x450 bpc=8 bpp=16 alphaInfo=.premultipliedLast (raw 1) colorSpace=Generic Gray Gamma 2.2

• We also compared the two decoded bitmaps pixel by pixel:
  • alpha channel: identical for every pixel
  • every semi-transparent edge pixel: byte values identical between A and B (none of them is brighter in B)
  • only fully-transparent pixels' color bytes differ (zeroed in B) — invisible either way

So the over-bright edge is not present in the decoded bytes themselves. At decode time, the visible difference between A and B is the CGImageAlphaInfo tag (straight vs premultiplied).

• On the same device, all of these render correctly:

  • UIImage(contentsOfFile:) / UIImage(data:) directly
  • converting the PNG to 8-bit sRGB RGBA first, then the same SDWebImage code path
  • re-rendering the SDWebImage result with UIGraphicsImageRenderer (preferredRange = .standard, transparency kept)
  • Kingfisher 7.9.1 with default options

• Our guess (may be wrong): the straight alpha flag is replaced with a premultiplied flag, so the semi-transparent white edge pixels get composited at full strength. We are not sure (informational only)

Environment

• SDWebImage 5.21.7 (CocoaPods)
• iPhone 13 Pro, iOS 26.6.2, real device
• UIImageView + sd_setImage(with:), default options (.retryFailed)

Thank you!