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XMED File Comparisons (Updated)

This note captures the byte-level differences observed when several sample CASt files are compared against the baseline Text_Hallo.cst. The raw tables from the original investigation are retained, but the surrounding commentary has been refreshed to reflect what the current parser understands about those bytes.

1. Header byte deltas (text members)

The table below lists the length delta and the first five differing offsets for each text sample relative to Text_Hallo.cst. Offsets are shown in hexadecimal and values are little-endian bytes.

File Length diff First diffs
ImgCast.cst length 4139 vs 3213 0x0004:86→24, 0x0005:0C→10, 0x003C:0F→11, 0x0050:86→24, 0x0051:0C→10
Text_12chars.cst length 7296 vs 3213 0x0004:86→78, 0x0005:0C→1C, 0x0018:2C→7A, 0x0019:00→16, 0x002C:70→2A
Text_Hallo2.cst length 6664 vs 3213 0x0004:86→00, 0x0005:0C→1A, 0x0018:2C→90, 0x0019:00→17
Text_Hallo_2line_linespace_16.cst length 7478 vs 3213 0x0004:86→F0, 0x0005:0C→1B, 0x0018:2C→B2, 0x0019:00→15, 0x002C:70→2A
Text_Hallo_2line_linespace_30.cst length 7654 vs 3213 0x0004:86→DE, 0x0005:0C→1D, 0x0018:2C→DE, 0x0019:00→05, 0x002C:70→2A
Text_Hallo_2line_linespace_Default.cst length 7478 vs 3213 0x0004:86→F0, 0x0005:0C→1B, 0x0018:2C→80, 0x0019:00→19, 0x002C:70→2A
Text_Hallo_NoBold.cst length 6704 vs 3213 0x0004:86→28, 0x0005:0C→1A, 0x0018:2C→B8, 0x0019:00→17
Text_Hallo_editable_true.cst length 7518 vs 3213 0x0004:86→56, 0x0005:0C→1D, 0x0018:2C→E6, 0x0019:00→1A, 0x002C:70→2A
Text_Hallo_font_Vivaldi.cst length 6886 vs 3213 0x0004:86→DE, 0x0005:0C→1A, 0x0018:2C→6E, 0x0019:00→18
Text_Hallo_fontsize14.cst length 7336 vs 3213 0x0004:86→58, 0x0005:0C→1C, 0x0018:2C→E8, 0x0019:00→19, 0x002C:70→2A
Text_Hallo_italic.cst length 7336 vs 3213 0x0004:86→A0, 0x0005:0C→1C, 0x0018:2C→30, 0x0019:00→1A, 0x002C:70→2A
Text_Hallo_letterSpace_6.cst length 6718 vs 3213 0x0004:86→36, 0x0005:0C→1A, 0x0018:2C→C6, 0x0019:00→17
Text_Hallo_multiLine.cst length 6846 vs 3213 0x0004:86→B6, 0x0005:0C→1A, 0x0018:2C→46, 0x0019:00→18
Text_Hallo_tab_true.cst length 7518 vs 3213 0x0004:86→56, 0x0005:0C→1D, 0x0018:2C→46, 0x0019:00→10, 0x002C:70→2A
Text_Hallo_textAlignLeft.cst length 7478 vs 3213 0x0004:86→2E, 0x0005:0C→1D, 0x0018:2C→BE, 0x0019:00→1A, 0x002C:70→2A
Text_Hallo_textAlignRight.cst length 7478 vs 3213 0x0004:86→2E, 0x0005:0C→1D, 0x0018:2C→D0, 0x0019:00→15, 0x002C:70→2A
Text_Hallo_underline.cst length 7336 vs 3213 0x0004:86→A0, 0x0005:0C→1C, 0x0018:2C→9C, 0x0019:00→16, 0x002C:70→2A
Text_Hallo_wrap_off.cst length 7518 vs 3213 0x0004:86→64, 0x0005:0C→1C, 0x0018:2C→F4, 0x0019:00→19, 0x002C:70→2A
Text_Hallo_changed_color.cst length 6730 vs 3213 0x0004:86→42, 0x0005:0C→1A, 0x0018:2C→D2, 0x0019:00→17
Text_Hallo_text_transform_all_on.cst length 7346 vs 3213 0x0004:86→AA, 0x0005:0C→1C, 0x0018:2C→3A, 0x0019:00→1A, 0x002C:70→2A
Text_Hallo_margin_spacing_FirstInd.cst length 6682 vs 3213 0x0004:86→12, 0x0005:0C→1A, 0x0018:2C→A2, 0x0019:00→17
Text_Hallo_multifont.cst length 7318 vs 3213 0x0004:86→8E, 0x0005:0C→1C, 0x0018:2C→1E, 0x0019:00→1A, 0x002C:70→2A
Text_Hallo_strikeout.cst length 6714 vs 3213 0x0004:86→32, 0x0005:0C→1A, 0x0018:2C→C2, 0x0019:00→17
Text_Hallo_subscript.cst length 7352 vs 3213 0x0004:86→B0, 0x0005:0C→1C, 0x0018:2C→40, 0x0019:00→1A, 0x002C:70→2A
Text_Hallo_superscript.cst length 6776 vs 3213 0x0004:86→70, 0x0005:0C→1A, 0x0018:2C→00, 0x0019:00→18
Text_Hallo_with_name.cst length 4982 vs 3213 0x0004:86→6E, 0x0005:0C→13, 0x0018:2C→FE, 0x0019:00→10
Text_Multi_Line_Multi_Style.cst length 11165 vs 3213 0x0004:86→96, 0x0005:0C→2B, 0x0018:2C→5E, 0x0019:00→07, 0x002C:70→2A
Text_Single_Line_Multi_Style.cst length 10904 vs 3213 0x0004:86→90, 0x0005:0C→2A, 0x0018:2C→6A, 0x0019:00→07, 0x002C:70→2A
Text_Wider_Width4.cst length 6642 vs 3213 0x0004:86→EA, 0x0005:0C→19, 0x0018:2C→7A, 0x0019:00→17

Interpreting the offsets

The recurring offsets line up with the CASt header structure documented in the xmedCode.pat pattern and test hints:

  • 0x0018 — field width (twips). Wider samples update this value accordingly.【F:Test/TestData/Legacy/Texts_Fields/xmedCode.pat†L99-L115】
  • 0x001C — default style bits (bold/italic/underline/etc.).【F:Test/TestData/Legacy/Texts_Fields/xmedCode.pat†L52-L69】
  • 0x001D — layout flags (alignment, wrap, tab enable).【F:Test/TestData/Legacy/Texts_Fields/xmedCode.pat†L70-L87】
  • 0x003C / 0x0040 / 0x004C — line spacing, base font size, and text length.
  • 0x04DA / 0x04DE / 0x04E2 — margins and first-line indent.

When the table above shows a change at 0x0019, it corresponds directly to the alignment/wrap flags: left/right justification, wrap off, or tab toggles now match the parser’s interpretation.

Text and font runs

The multi-style samples (Text_Multi_Line_Multi_Style*) expose the run maps that BlXmedTokenParser now consumes. The font sequence observed during the original investigation (“Arcade *” → “Trajan Pro” → “Arcade *”) is still visible; the modern parser resolves it through the Fonts block and assigns the resulting family/style names to each XmedStyleDescriptor.【F:src/BlingoEngine.IO.Legacy/Texts/BlXmedTokenStyleParser.cs†L197-L215】

2. Field cast comparisons

Raw byte differences for field members are preserved below. These files reuse the same header layout as text members, so the offset interpretations from the previous section apply here as well.

| Field_Hallo2.cst | same | 0x0004:BE→4C, 0x0005:14→0E, 0x0018:4E→F0, 0x0019:12→06, 0x004C:08→0D | | Field_Hallo_3lines.cst | same | 0x0004:BE→4C, 0x0005:14→0E, 0x0018:4E→DC, 0x0019:12→0B, 0x004C:08→09 | | Field_Hallo_align_center.cst | same | 0x0004:BE→A4, 0x0005:14→0B, 0x0018:4E→34, 0x0019:12→09, 0x004C:08→04 | | Field_Hallo_align_right.cst | same | 0x0004:BE→A4, 0x0005:14→0B, 0x0018:4E→BC, 0x0019:12→06, 0x004C:08→04 |

3. Colour experiments

Colour-altered casts (Text_Hallo_changed_color.cst, Text_Hallo_text_transform_all_on.cst) still show the FFFF0000000600040001 pattern at the start of the colour table. Later segments (e.g. 01CC00 01FF00 016600) encode the RGB components that the style parser now lifts into XmedStyleDescriptor.ForegroundColor.【F:src/BlingoEngine.IO.Legacy/Texts/BlXmedTokenStyleParser.cs†L92-L123】 The palette indices logged as 02:5B, 02:5C, etc., map to Director’s Fore/Back colour ids and remain visible in the header group (03:FFFF…).

4. Historical scratchpad (kept for reference)

The original investigation contained several speculative notes. They are kept here with inline corrections so future work can trace the evolution of the understanding:

  • Directory rows as <TYPE,OFFSET,COUNT> – offsets are not present. The parser derives block boundaries from the ASCII counts instead.
  • 00(40): font name heuristic – the pattern correctly spots font entries, but the follow-up block is part of the structured descriptor and not free-form padding.
  • Style flag bits – the guessed meanings (bold/italic/underline/strikeout/ subscript/superscript) align with the parser, but the “editable field” flag is stored in the CASt header rather than the style descriptor.
  • Control byte glossary (01, 02, 03, 81, 82, C1, C2, C3) – the tokenizer still emits the same values, yet the parser now maps them to concrete structures (style records, tab stops, spacing blocks). These annotations document where the research journey started; the corrected sections above describe how the engine currently interprets the data.

The alignment and style differences for left/right variants modify the bytes at offsets 0x18 and 0x19 in both formats, indicating these properties are stored in the same location.

Suspected text length byte

Field variants hint that the byte at offset 0x004C may hold a text length value. The table below lists this byte for a few sample files.

File Value at 0x004C
Field_Hallo.cst 0x08
Field_Hallo2.cst 0x0D
Field_Hallo_3lines.cst 0x09
Text_Hallo.cst XFIR
Text_Hallo2.cst XFIR