[Lakritzator]

Summary

WicFrameInfo.HasResolution: true when the file itself stores a resolution, false when DpiX/DpiY are only the default of the decoder. The rules per container are in Internal/ResolutionProbe.cs:

Container	Rule
PNG	/pHYs/UnitSpecifier through the query reader, otherwise a scan of the chunk headers up to IDAT; unit 1 (meter) → true, unit 0 or no pHYs → false
JPEG	JFIF /app0/{ushort=1} 1 or 2, or EXIF /app1/ifd/{ushort=282} with {ushort=296} not 1 (missing = inch)
TIFF, DNG	/ifd/{ushort=282} with 296 not 1
JPEG XR	WIDTH_RESOLUTION (0xBC82, T.832) in the container IFD, read from the file
BMP	biXPelsPerMeter (byte 38) not 0, BITMAPINFOHEADER or larger
GIF, ICO, DDS	false
others	an IFD block (found by its metadata format in the enumerated items) with XResolution and ResolutionUnit not 1

It's filled by GetFrameInfo and every Decode overload from the frame that is open anyway. The stream position is restored after the few byte reads.

Found on CI: the WIC JPEG decoder ignores the EXIF resolution. It reported 96 for a JPEG with only EXIF XResolution 72. When the resolution comes only from EXIF (JPEG without JFIF density, and the "others" rule), DpiX/DpiY now carry the EXIF values (unit cm converted), so HasResolution == true always comes with the file's own DPI.

Palette for Encode

• WicEncodeOptions.Palette (≤ 256 colors as 0xAARRGGBB, alpha 0 = transparent), WicEncodeOptions.Dither (error diffusion), and WicPixelFormats.Format1bppIndexed to Format8bppIndexed.
• Indexed pixels use the given palette (the source answers CopyPalette). Other pixels with a palette ask the encoder for 8bpp indexed and get the nearest color.
• GIF without a palette: at most 256 colors and no partial alpha → exactly those colors; otherwise InitializeFromBitmap as before.
• Also found on CI: the WIC format converter maps to a custom palette approximately (an exact color came out as a neighbor). The mapping is now done by WicIndexingPixelSource (exact match, otherwise nearest RGB, alpha < 128 → transparent entry). The WIC converter is only used for dithering.

Docs (resolution table, palette), changelog [3.19.0] and version 3.19.

Tests

WicResolutionTests: the files are made in the test (WIC encode, then the PNG/JPEG/BMP resolution fields are patched in the bytes, PNG with CRC). It covers PNG pHYs 72 / none / unit 0, JPEG JFIF inch / units 0 / EXIF-only 72 / EXIF unit none, TIFF, BMP 3780 and 0, GIF, JPEG XR, WebP (a 1x1 VP8L, skipped without the WebP codec), the Decode overloads, a stream at an offset, the GIF round trip with 256 colors, indexed 8bpp with a palette to GIF/PNG/BMP, nearest color, and > 256 colors with and without dither.

CI: net10 634 passed / 4 skipped, net48 636 passed / 4 skipped. The skips are HEIF, JPEG XL, WebP and a UI Automation test.

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[coveralls]

Coverage Report for CI Build 41463335738

Coverage increased (+0.7%) to 54.961%

Details

• Coverage increased (+0.7%) from the base build.
• Patch coverage: 61 uncovered changes across 3 files (282 of 343 lines covered, 82.22%).
• No coverage regressions found.

Uncovered Changes

File	Changed	Covered	%
src/Dapplo.Windows.Imaging/Internal/ResolutionProbe.cs	168	132	78.57%
src/Dapplo.Windows.Imaging/Internal/WicPixelSource.cs	64	49	76.56%
src/Dapplo.Windows.Imaging/WicImage.cs	102	92	90.2%
Total (6 files)	343	282	82.22%

Coverage Regressions

No coverage regressions found.

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Coverage Stats

Relevant Lines:	11741
Covered Lines:	6667
Line Coverage:	56.78%
Relevant Branches:	6512
Covered Branches:	3365
Branch Coverage:	51.67%
Branches in Coverage %:	Yes
Coverage Strength:	1053.37 hits per line

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💛 - Coveralls