destogod/README.md

DestoGod

A replacement for TP5, the 2012 Windows program used to program the LED destination signs on Yutong buses.

dist/DestoGod.html is the whole application — one file, no installer, no internet. Double-click it and it opens in any browser.

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Why this exists

TP5 is a 417 KB Chinese MFC application from November 2012, supplied by Guangzhou Tongda with the signs Yutong fits to its buses. It has no live preview, so building a destination list is guess-and-check, and the workflow is Windows-only.

The bus never talks to TP5. The sign controller reads a .td5 file off an SD card, and that is the entire interface. So a replacement does not need to touch any hardware or protocol — it only has to write byte-correct .td5 files.

That is what this does.

What it does

• Opens the files the company already has. Drop in a .td5 (what goes on the SD card) or a .tp5 (the TP5 project). Drop in both and it uses the text from the project with the exact artwork from the bus file.
• Live sign preview. The message is drawn as an LED matrix at the real size of the real sign, and scrolls exactly as it will on the bus. TP5 shows you nothing until you export.
• Tells you when a message will not fit and by how many pixels.
• Imports a spreadsheet. Drop in an .xlsx (or CSV) and it reads the Line | Line Name | Description | Content (Display) layout of the template these buses ship with — Line Name becomes what the driver sees on the controller, Content (Display) becomes what the sign shows. Names that already exist are updated rather than duplicated. No library involved: the .xlsx is unzipped and parsed by the browser itself.
• Paste a whole list at once instead of typing destinations one at a time.
• Delete a destination with the × on its row in the list, or clear the whole list at once. Both offer an Undo instead of asking you to confirm first — a confirm box is friction while tidying a list and still leaves you stuck if you confirm by mistake.
• Refuses to ship a blank destination. Tapping "+" one time too many leaves an entry with nothing in it; those are flagged in the list and the export stops with a one-click "Remove and export". A new destination starts genuinely empty rather than pre-filled with placeholder text, because placeholder text is worse than blank here — "NEW DESTINATION" will happily print itself on the front of a bus.
• Keeps the original artwork for any destination you have not edited, so re-exporting an existing list changes nothing about it.
• Exports .td5 for the SD card, and .tp5 so the old software can still open the list during the changeover.

How the file formats were worked out

Everything below was derived from the sample files and from TP5(En).exe itself, then verified byte-for-byte.

.td5 — the file the bus reads

FILE
  0x000  16  magic, GBK "广州通达图形线路"
  0x010   4  "V5.0"
  0x018  12  export timestamp, ASCII YYMMDDhhmmss
  0x030   2  company count
  0x040   2  sign height, sign width in bytes   (16, 14 = 112x16)
  0x080  16  company name           \ repeats every 0x20 per company
  0x090   6  pointer-table offset, destination count
  0x200  ..  u32 offset per destination, terminated by 0xffffffff
  0x400  ..  destination records; unused space is filled with 0xff

DESTINATION   record header is 0x80 bytes; its block always starts at +0x200
  +0x00  16  name
  +0x10  16  name again — THIS is the copy the driver's controller displays,
             and TP5 always writes it identical to the first (verified across
             all 36 records). If the two are allowed to differ, the sign shows
             the right words but the controller shows the wrong ones. See
             setName() in app/main.mjs; export forces them equal, and opening
             a file repairs a pair that has already drifted.
  +0x30   8  block offset, block length
  records are padded up to the next 0x200 boundary

BLOCK
  +0x00   1  0x43 'C'
  +0x01   2  CRC-16/ARC over block[3 .. blockLength]
  +0x03   4  block id — TP5 stores rand(), so it never exceeds 0x7fff
  +0x07   4  block length
  +0x0b   3  frame count, three times
  +0x0f   1  0x84
  +0x10   6  per frame: u32 bitmap offset, u8 width in bytes, u8 height
         ..  two 0x80-byte action sections per frame
         ..  the frame bitmaps
  header length = 0x10 + frames*6 + frames*0x100

BITMAP  one 16-byte chunk per 8 pixels of width; each chunk is 8 columns x 16
        rows, row-major, high bit leftmost. Wider than the sign means it scrolls.

The checksum was the last piece. Bytes +5 and +6 are always zero, which hid a field boundary: +1 is a 2-byte CRC and +3 is a 4-byte random id whose top half is always zero because RAND_MAX is 0x7fff. The CRC is CRC-16/ARC (reflected, polynomial 0xa001, init 0), taken over everything from +3 to the end of the block. TP5 computes it in sub_409150, picking polynomial index 1 out of the table at 0x447020 = {0x8480, 0xa001, 0x8621, 0xe950}, and stores it in SetTypeAndCrc at 0x4353c0.

Because the id is random and sits inside the CRC's range, two exports of the same list are never byte-identical — but any id you choose is valid.

.tp5 — the editable project

Line-based text, CRLF, one statement per line. Strings are UTF-16BE hex. Each destination has five screens (front, rear, side, inner, backside), each with "up" and "down" action lists, each action holding five icons that can carry text objects. Only the front screen is used in practice.

.font — the sign's built-in fonts

name, then cellWidth,height, then per glyph: character code, advance width, and height * ceil(cellWidth/8) bytes — each byte XORed with the character code. All 18 shipped fonts are included in the app.

Rendering to match the existing signs

The company's current signs were made with the Windows font Impact, which TP5 hands to GDI. A browser canvas rasterises the same font differently, so the renderer is calibrated against the real output: TP5's size 20 corresponds to 15.77 canvas pixels (scale 0.789, measured across all 36 destinations), and the alpha threshold is 225, because GDI lays down far less antialiasing than canvas does. At those settings the rendered stroke weight, width and height all land within 2% of TP5's own output.

Those numbers were measured on a Mac, and macOS, Windows and Linux all rasterise fonts slightly differently — so they are only the starting point. When both a .tp5 and a .td5 are loaded the app knows the text and the exact artwork TP5 produced for it, so it solves for the size and threshold that reproduce that artwork on whatever computer it is running on, and says so in the preview. On the development Mac it settles on 0.804 / 245 — a closer match than the hand-tuned constants. Nothing about this depends on the machine being the one it was written on.

Which computers it runs on

Any reasonably current browser — Edge, Chrome, Brave or Firefox — on Windows, macOS or Linux. Edge is already on every Windows 10/11 machine, so on a depot PC there is nothing to install. Verified opening straight off the disk with no server (file://).

Two things are worth knowing:

• Nothing about the exported file depends on the operating system. The .td5 writer is pure integer work: same list in, same bytes out, on any machine. The sign's own built-in fonts are bitmaps, so they are identical everywhere too. Only the system-font (Impact) path touches the OS rasteriser, and that is what the calibration above corrects for.
• Impact ships with Windows, and always has. It is the font TP5 was using via GDI, so on a Windows machine the lettering has a shorter distance to travel than it did here.

The floor is a browser from roughly 2020 (Chrome/Edge 80+, Firefox 75+). It deliberately avoids the newest APIs — no structuredClone, and it falls back gracefully if <dialog> is unsupported — so an older depot PC is fine.

Verification

node test/roundtrip.mjs      # re-build a real TP5 export and compare byte-for-byte
node test/tp5roundtrip.mjs   # same for the project format
node test/authoring.mjs      # build a file from scratch, read it back, check CRCs
node test/sheet.mjs          # spreadsheet column mapping
node test/fontpreview.mjs "SCHOOL BUS"   # render sample text in every built-in font

roundtrip.mjs parses the real EXPRESS 1.td5, rebuilds all 46,080 bytes from the parsed model with a recomputed CRC, and requires an exact match. All 36 stored CRCs verify against the implementation.

Driven through its own interface — open the project, merge the artwork, press Export — the app reproduces the company's real file with 7 differing bytes, all of them the export timestamp, and no other difference anywhere.

Retyping a single destination and exporting again changes only that destination's block (plus the timestamp). The other 35 stay byte-identical, so editing one entry cannot disturb the rest of the list.

On the road

DestoGod files have been loaded onto a real bus. The signs display correctly and the driver's controller lists the destinations correctly — the format is confirmed against the actual hardware, not just against TP5's output.

The first road test found the one thing no amount of byte-comparison would have caught: the destination name is stored twice, and the driver's controller reads the second copy. Renaming a destination updated only the first, so the signs were right while the controller showed nothing useful. Fixed, forced to stay in step on both export and import, and confirmed on the bus afterwards.

Still worth doing when convenient: export the same list from DestoGod and from TP5 and compare. They should differ only in the timestamp and the random block ids (and the CRCs that follow from them).

What is on the SD card

Only the .td5. TP5 also leaves a folder tree next to it — <export name>/<company>/<destination>/, one folder per destination — but every one of those 36 folders is empty, so it is scratch output, not something the controller reads. Checked directly rather than assumed, because it was the obvious suspect when the controller names went missing.

Building

node tools/bundle.mjs        # everything   -> dist/DestoGod.html

src/fonts.js is committed, so a fresh clone builds and runs without anything else. Regenerating it needs the original .font files that ship beside TP5, which are not in this repo:

node tools/build-fonts.mjs   # ../../yutongapp/*.font -> src/fonts.js

Layout

src/codec/td5.mjs      read and write the file the bus reads
src/codec/tp5.mjs      read and write the TP5 project file
src/codec/bitfont.mjs  the sign's built-in bitmap fonts
src/codec/sheet.mjs    .xlsx / .csv reading, no dependencies
src/fonts.js           generated from the 18 shipped .font files
app/                   the interface
tools/                 font conversion, single-file bundler
test/                  the checks described above
re/                    the reverse-engineering scripts, kept as working notes
dist/DestoGod.html     ← the thing to actually use