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Add ImageInfo.CanDecode and UnsupportedImageFeatureException for PNG Adam7 interlacing - #18
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…Adam7 interlacing Adds ImageInfo.CanDecode (init-only, default true) so GetInfo can report that a well-formed file declares a feature Load cannot decode, without requiring a caller to call Load and catch an exception just to find out. PNG is currently the only codec where this applies: Adam7-interlaced files are well-formed PNG per the specification, but PngCodec.Load has no interlaced-scanline reconstruction logic. Adds UnsupportedImageFeatureException, thrown by PngCodec.Load instead of InvalidDataException for the Adam7 case, so callers can distinguish well-formed-but-unsupported from malformed input without string-matching Exception.Message. Because System.IO.InvalidDataException is sealed, this new type derives from IOException (InvalidDataException's own base) instead - a deliberate, documented behavior change: an existing catch (InvalidDataException) around PngCodec.Load no longer catches the Adam7 case. - ImageInfo.cs: add CanDecode as an init-only property outside the primary constructor, to avoid a binary-incompatible change to the record struct's generated constructor/Deconstruct signature. - UnsupportedImageFeatureException.cs (new): sealed exception type with a Feature token identifying the specific unsupported feature. - PngCodec.cs: ParseIhdr/PngHeader carry InterlaceMethod; Load throws UnsupportedImageFeatureException for Adam7; GetInfo sets CanDecode accordingly. - Tests: renamed/added PngCodec and PngSuite tests for the new exception type and CanDecode; new ImageInfoTests.cs and UnsupportedImageFeatureExceptionTests.cs. - Companion artifacts: reqstream/design/verification docs, README, user guide, and .reviewmark.yaml updated to match. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
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Copilot review overview
🟡 Changes recommended
Unresolved moderate issues remain in exception guidance and ImageInfo default-state behavior.
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Review effort: Lite
Findings: 5
Open (8)
What changed in this PR
Adds PNG Adam7 capability detection via ImageInfo.CanDecode and a dedicated unsupported-feature exception.
Changes:
- Adds Adam7 detection and
UnsupportedImageFeatureException. - Updates PNG tests and conformance expectations.
- Updates documentation, requirements, verification, and traceability mappings.
Review identified unresolved API/documentation and default-value issues.
| File | Summary |
|---|---|
test/DemaConsulting.CanvasNet.Tests/Codecs/UnsupportedImageFeatureExceptionTests.cs |
Tests the new exception behavior. |
test/DemaConsulting.CanvasNet.Tests/Codecs/PngSuiteTests.cs |
Updates PNG conformance expectations. |
test/DemaConsulting.CanvasNet.Tests/Codecs/PngCodecTests.cs |
Tests Adam7 handling. |
test/DemaConsulting.CanvasNet.Tests/Codecs/ImageInfoTests.cs |
Tests CanDecode. |
src/DemaConsulting.CanvasNet/Codecs/UnsupportedImageFeatureException.cs |
Defines the unsupported-feature exception. |
src/DemaConsulting.CanvasNet/Codecs/PngCodec.cs |
Detects Adam7 and reports decode capability. |
src/DemaConsulting.CanvasNet/Codecs/ImageInfo.cs |
Adds decode-capability metadata. |
README.md |
Documents pre-decode checks. |
docs/verification/canvas-net/codecs/png-codec.md |
Updates PNG verification records. |
docs/user_guide/introduction.md |
Documents API behavior and usage. |
docs/reqstream/canvas-net/codecs/png-codec.yaml |
Updates PNG requirements. |
docs/reqstream/canvas-net/codecs.yaml |
Updates codec requirements. |
docs/reqstream/canvas-net.yaml |
Updates system requirements. |
docs/design/canvas-net/codecs/png-codec.md |
Updates PNG design details. |
docs/design/canvas-net/codecs.md |
Updates shared codec design guidance. |
docs/design/canvas-net.md |
Updates public API design documentation. |
.reviewmark.yaml |
Registers reviewed artifacts. |
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…from review - Correct the repeated claim that IOException is a common base for InvalidDataException and UnsupportedImageFeatureException: InvalidDataException derives directly from SystemException, not IOException. A caller that wants to handle both malformed (InvalidDataException) and well-formed-but-unsupported (UnsupportedImageFeatureException) cases together must add two explicit catch clauses, not a single catch (IOException). Fixed in UnsupportedImageFeatureException.cs, docs/design/canvas-net/codecs.md, and docs/user_guide/introduction.md. - Document that default(ImageInfo) (or an uninitialized array element) has CanDecode == false, not true, because a struct's property initializers only run when a declared constructor runs; default(ImageInfo) zero-initializes every field directly. Added ImageInfoTests.ImageInfo_DefaultValue_HasCanDecodeFalse to cover this explicitly. - PngCodec.cs: add the missing UnsupportedImageFeatureException documentation to the Load(string path) overload, and correct the validateDecodability parameter doc on the private ParseIhdr helper (said InvalidDataException, throws UnsupportedImageFeatureException). - README.md: use the feature-and-message UnsupportedImageFeatureException constructor overload in the untrusted-file-triage example, so the example preserves the Feature token instead of discarding it. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
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Copilot review overview
🔵 Needs a closer look
One or more issues must be addressed before approval.
Review effort: Lite
Findings: None
Resolved since last review (8)
This was referenced Oct 16, 2026
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Summary
Implements Part 1 of the signed-off feedback-driven backlog (well-formed-but-unsupported PNG detection).
Why IOException and not InvalidDataException?
System.IO.InvalidDataException is sealed in .NET, so a new exception type cannot derive from it. UnsupportedImageFeatureException instead derives from IOException, InvalidDataException's own base class.
This is a deliberate, narrow, documented behavior change: an existing catch (InvalidDataException) around PngCodec.Load will no longer catch the Adam7-interlacing case. Callers that want to handle both malformed and well-formed-but-unsupported input together should catch IOException, or add an explicit catch (UnsupportedImageFeatureException).
Scope
Scoped to PNG only. Investigation across all four raster codecs (BMP/PNG/TIFF/JPEG) found Adam7 interlacing is the only case where a codec's Load refuses a file that is otherwise well-formed per its format's own specification; the others already conflate malformed/unsupported at GetInfo time.
Changes
Testing
Co-authored-by: Copilot 223556219+Copilot@users.noreply.github.com