Add Vedic mark rendering prototype for Baloo Tammudu 2 (Telugu)

[revurpk]

This is a prototype for review generated using Claude Code, not a merge-ready implementation. The TODO list in the README captures what's still needed.

Adds rendering support for Vedic accent marks to all five weights of Baloo Tammudu 2 by grafting outlines from Noto Sans Devanagari (OFL) into the released TTX artefacts, then wiring proper OpenType layout.

Codepoints covered (45 total):

• U+0951-U+0954 Devanagari Vedic accents (udatta, anudatta, grave, acute) - widely used in Telugu Sanskrit prosody
• U+1CD0-U+1CFF Unicode Vedic Extensions block (41/48 assigned; gaps are Unicode 12/13 additions or unassigned codepoints that the donor does not have either)

Implementation

vedic-build/graft_vedic.py

• Flattens donor composites via DecomposingRecordingPen.
• Adapts geometry: above-marks have bbox-bottom aligned to Y=600 (Baloos existing mark band); all marks centered at X=-300 so they land over a typical Telugu base under static positioning.
• Sets zero advance width, GDEF class 3 (mark), and adds cmap entries on the Windows BMP subtable.

vedic-build/add_vedic_gpos.py

• Adds a new GPOS lookup (type 4, MarkBasePos) with anchors on 74 Telugu consonants (top-center at ymax + 220) and 27 Vedic above-marks (bottom-center at ymin).
• Excludes above-base matras and halant from the base coverage so the mark attaches to the consonant directly rather than to a matras off-center bbox.
• Registers a new abvm feature on the tel2 and telu scripts.

Build pipeline

TTX -> intermediate TTF (via fontTools.ttx)
-> patched TTF (via graft_vedic.py)
-> patched TTF (via add_vedic_gpos.py adds GPOS)

The five patched TTFs land in BalooTammudu2-Telugu/vedic-build/ and are gitignored (covered by repo-wide *.ttf rule); regenerate with the pipeline documented in vedic-build/README.md.

Scope and caveats

This is a non-authoritative prototype. The .vfb sources in BalooTammudu2-Telugu/*/VFB/ are unchanged. A feature request tracking the proper work has been filed at github.com/EkType/Baloo2/issues.

Limitations of the graft:

• Style mismatch: Notos stroke contrast differs from Baloos.
• Only above-marks get GPOS anchors; below-marks (U+0952 anudatta and a few in Vedic Extensions) rely on static positioning.
• Tall above-base matras like matraIi come within ~80 units of the marks; tune BASE_ANCHOR_Y_GAP in add_vedic_gpos.py for more clearance.

TODO for the proper fix (full list in vedic-build/README.md):

• Draw native Vedic glyphs in FontLab matched to Baloos display style, in all five weights. This is the blocker.
• Add GlyphOrderAndAliasDB and GDEF entries for the new glyphs.
• Design real GPOS anchors (above and below) in the .vfb sources rather than the heuristic this prototype uses.
• Handle above-base matras via mkmk anchors or contextual GPOS instead of the current exclude-from-coverage workaround.
• Register abvm/blwm features in the source feature tables so the fix survives the next VFB->TTF rebuild (this prototype patches GPOS at the TTF level only).
• Cover the seven gaps the donor lacks (U+1CF7, U+1CFA).
• Visual QA across a Vedic test corpus.
• Remove vedic-build/ once proper sources ship.

Files changed

OFL.txt Noto attribution per OFL S4
BalooTammudu2-Telugu/vedic-build/.gitignore Skip donor tree, ephemera
BalooTammudu2-Telugu/vedic-build/README.md Pipeline, caveats, TODO
BalooTammudu2-Telugu/vedic-build/graft_vedic.py
BalooTammudu2-Telugu/vedic-build/add_vedic_gpos.py