Feature/svg gradient support

[Ani07-05]

Fixes #1580

This PR implements comprehensive SVG gradient support for fpdf2, enabling rendering of both linear and radial gradients from SVG documents.

Features

• Linear gradients with all SVG coordinate systems
• Radial gradients with focal point (fx, fy, fr) support
• Gradient stops with color and opacity
• Spread methods: pad, reflect, repeat (related to PR COLRv1 support enhancements #1636)
• Gradient transforms (rotation, scaling, translation)
• Support for both objectBoundingBox and userSpaceOnUse units
• Gradients can be applied to both fill and stroke

Implementation

• Added gradient parsing methods in SVGObject class
• Integrated with existing fpdf2 GradientPaint system
• Gradients stored in gradient_definitions dict by ID
• Applied via url(#gradientId) references in fill/stroke

Tests

• 18 comprehensive test cases covering all gradient features
• All 287 SVG tests passing (18 new + 269 existing)
• Added 5 SVG test fixtures and 5 PDF reference files

Checklist:

• A unit test is covering the code added / modified by this PR
• In case of a new feature, docstrings have been added, with also some documentation in the docs/ folder
• A mention of the change is present in CHANGELOG.md
• This PR is ready to be merged

By submitting this pull request, I confirm that my contribution is made under the terms of the GNU LGPL 3.0 license.

[Ani07-05]

@andersonhc @Lucas-C please review and let me know if there are any changes.

[andersonhc]

Please run pylint on your code before the next commit. Those are the errors I'm getting:

************* Module fpdf.svg
fpdf/svg.py:901:16: W1203: Use lazy % formatting in logging functions (logging-fstring-interpolation)
fpdf/svg.py:885:4: R6301: Method could be a function (no-self-use)
fpdf/svg.py:941:24: W1203: Use lazy % formatting in logging functions (logging-fstring-interpolation)
fpdf/svg.py:968:16: W1203: Use lazy % formatting in logging functions (logging-fstring-interpolation)
fpdf/svg.py:907:4: R6301: Method could be a function (no-self-use)
fpdf/svg.py:974:4: R6301: Method could be a function (no-self-use)
fpdf/svg.py:1017:12: W1203: Use lazy % formatting in logging functions (logging-fstring-interpolation)
fpdf/svg.py:1024:19: W0718: Catching too general exception Exception (broad-exception-caught)
fpdf/svg.py:1025:16: W1203: Use lazy % formatting in logging functions (logging-fstring-interpolation)
fpdf/svg.py:1030:12: W1203: Use lazy % formatting in logging functions (logging-fstring-interpolation)
fpdf/svg.py:1051:8: W1203: Use lazy % formatting in logging functions (logging-fstring-interpolation)
fpdf/svg.py:1079:12: W1203: Use lazy % formatting in logging functions (logging-fstring-interpolation)
fpdf/svg.py:1102:19: W0718: Catching too general exception Exception (broad-exception-caught)
fpdf/svg.py:1103:16: W1203: Use lazy % formatting in logging functions (logging-fstring-interpolation)
fpdf/svg.py:1107:12: W1203: Use lazy % formatting in logging functions (logging-fstring-interpolation)
fpdf/svg.py:1129:8: W1203: Use lazy % formatting in logging functions (logging-fstring-interpolation)
fpdf/svg.py:1140:16: W1203: Use lazy % formatting in logging functions (logging-fstring-interpolation)
fpdf/svg.py:1148:16: W1203: Use lazy % formatting in logging functions (logging-fstring-interpolation)
************* Module test.svg.test_svg_gradients
test/svg/test_svg_gradients.py:27:8: C0415: Import outside toplevel (fpdf.drawing.GradientPaint) (import-outside-toplevel)
test/svg/test_svg_gradients.py:9:4: R6301: Method could be a function (no-self-use)
test/svg/test_svg_gradients.py:31:4: R6301: Method could be a function (no-self-use)
test/svg/test_svg_gradients.py:49:4: R6301: Method could be a function (no-self-use)
test/svg/test_svg_gradients.py:72:4: R6301: Method could be a function (no-self-use)
test/svg/test_svg_gradients.py:93:4: R6301: Method could be a function (no-self-use)
test/svg/test_svg_gradients.py:124:8: C0415: Import outside toplevel (fpdf.enums.GradientUnits) (import-outside-toplevel)
test/svg/test_svg_gradients.py:109:4: R6301: Method could be a function (no-self-use)
test/svg/test_svg_gradients.py:145:8: C0415: Import outside toplevel (fpdf.enums.GradientUnits) (import-outside-toplevel)
test/svg/test_svg_gradients.py:131:4: R6301: Method could be a function (no-self-use)
test/svg/test_svg_gradients.py:152:4: R6301: Method could be a function (no-self-use)
test/svg/test_svg_gradients.py:170:4: R6301: Method could be a function (no-self-use)
test/svg/test_svg_gradients.py:187:4: R6301: Method could be a function (no-self-use)
test/svg/test_svg_gradients.py:203:4: R6301: Method could be a function (no-self-use)
test/svg/test_svg_gradients.py:219:4: R6301: Method could be a function (no-self-use)
test/svg/test_svg_gradients.py:235:4: R6301: Method could be a function (no-self-use)
test/svg/test_svg_gradients.py:252:4: R6301: Method could be a function (no-self-use)
test/svg/test_svg_gradients.py:266:4: R6301: Method could be a function (no-self-use)
test/svg/test_svg_gradients.py:282:4: R6301: Method could be a function (no-self-use)
test/svg/test_svg_gradients.py:299:4: R6301: Method could be a function (no-self-use)
test/svg/test_svg_gradients.py:315:4: R6301: Method could be a function (no-self-use)
test/svg/test_svg_gradients.py:2:0: W0611: Unused FPDF imported from fpdf (unused-import)

[Ani07-05]

@andersonhc I have updated my files as per your suggestions please let me know if they require further corrections

[Ani07-05]

@andersonhc any updates and suggestions?

[andersonhc]

Please see comments.

Also pylint errors:
************* Module fpdf.svg
fpdf/svg.py:885:4: R6301: Method could be a function (no-self-use)
fpdf/svg.py:907:4: R6301: Method could be a function (no-self-use)
fpdf/svg.py:974:4: R6301: Method could be a function (no-self-use)
fpdf/svg.py:1024:19: W0718: Catching too general exception Exception (broad-exception-caught)
fpdf/svg.py:1102:19: W0718: Catching too general exception Exception (broad-exception-caught)

and one test is failing on `pytest`:

=========================== short test summary info ============================
FAILED test/image/test_vector_image.py::test_svg_image_with_unsupported_color - Failed: DID NOT RAISE <class 'ValueError'>

[Ani07-05]

hey @andersonhc All feedback addressed! Here's what I did:

1. Fixed all pylint errors:

   • Converted _convert_gradient_coordinate(), _parse_gradient_stops(), and _extract_gradient_id() to static methods (fixes R6301: no-self-use)
   • Replaced broad Exception catches with specific exception types (ValueError, AttributeError, TypeError) (fixes W0718: broad-exception-caught)

2. Fixed failing test:

   • Updated test_svg_image_with_unsupported_color to test actual unsupported colors (HSL format) instead of gradients, which are now supported
   • Test now passes successfully

3. Updated documentation:

   • Removed the detailed "Gradients" subsection from SVG.md as you requested
   • Kept only the bullet point in the features list for consistency with other SVG features

4. Updated CHANGELOG:

   • Rebased on latest master
   • Moved the SVG gradient entry to[2.8.6] section as requested

All tests passing, no pylint errors, and branch is now up to date with master.

[andersonhc]

@allcontributors please add @Ani07-05 for code

[allcontributors]

@andersonhc

I've put up a pull request to add @Ani07-05! 🎉