custom writer to Excalidraw?

[bapt a <auguieba-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

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Hi,

I'd like to extract all math equations from my lecture notes (and 
potentially images too, at some stage) and produce a json file suitable for 
Excalidraw, so that students could reorganise them, sketch notes 
collaboratively, etc.:

https://math.preview.excalidraw.com/

[image: Screenshot 2023-01-26 at 20.34.57.png]

(note that this is using a user-contributed extension*; the plain version 
of Excalidraw does not support Mathjax or Asciimath). 

From an equation like this in the source document (beamer tex):

```
\[\begin{aligned}
\nabla\cdot\mathbf{D}&= \rho_f\\
\nabla\cdot\mathbf{B}&= 0\\
\nabla\times\mathbf{E}&= -\frac{\partial\mathbf{B}}{\partial t}\\
\nabla\times\mathbf{H}&= \frac{\partial\mathbf{D}}{\partial t} + 
\mathbf{J}_f
\end{aligned}\]
```

the full json file suitable for import in Excalidraw looks like:

```
{
"type": "excalidraw",
"version": 2,
"source": "https://math.preview.excalidraw.com",
"elements": [
{
"type": "text",
"version": 104,
"versionNonce": 669974443,
"isDeleted": false,
"id": "iL0y7olLhWzLKVTzIffR1",
"fillStyle": "hachure",
"strokeWidth": 1,
"strokeStyle": "solid",
"roughness": 1,
"opacity": 100,
"angle": 0,
"x": 418,
"y": -186,
"strokeColor": "#000000",
"backgroundColor": "transparent",
"width": 202,
"height": 169,
"seed": 1021462275,
"groupIds": [],
"roundness": null,
"boundElements": [],
"updated": 1674718443076,
"link": null,
"locked": false,
"subtype": "math",
"customData": {
"useTex": true,
"mathOnly": true,
"ariaLabel": "\\begin{aligned} \\nabla\\cdot\\mathbf{D}&= \\rho_f\\\\ \\
nabla\\cdot\\mathbf{B}&= 0\\\\ \\nabla\\times\\mathbf{E}&= -\\frac{\\partial
\\mathbf{B}}{\\partial t}\\\\ \\nabla\\times\\mathbf{H}&= \\frac{\\partial\\
mathbf{D}}{\\partial t} + \\mathbf{J}_f \\end{aligned}"
},
"fontSize": 20,
"fontFamily": 2,
"text": "\\begin{aligned}\n\\nabla\\cdot\\mathbf{D}&= \\rho_f\\\\\n\\nabla\\
cdot\\mathbf{B}&= 0\\\\\n\\nabla\\times\\mathbf{E}&= -\\frac{\\partial\\
mathbf{B}}{\\partial t}\\\\\n\\nabla\\times\\mathbf{H}&= \\frac{\\partial\\
mathbf{D}}{\\partial t} + \\mathbf{J}_f\n\\end{aligned}",
"baseline": 91,
"textAlign": "left",
"verticalAlign": "top",
"containerId": null,
"originalText": "\\begin{aligned}\n\\nabla\\cdot\\mathbf{D}&= \\rho_f
\\\\\n\\nabla\\cdot\\mathbf{B}&= 0\\\\\n\\nabla\\times\\mathbf{E}&= -\\frac{
\\partial\\mathbf{B}}{\\partial t}\\\\\n\\nabla\\times\\mathbf{H}&= \\frac{
\\partial\\mathbf{D}}{\\partial t} + \\mathbf{J}_f\n\\end{aligned}"
},
{
"type": "text",
"version": 111,
"versionNonce": 1291434347,
"isDeleted": false,
"id": "xzGehwC0igI1rh-jYddgb",
"fillStyle": "hachure",
"strokeWidth": 1,
"strokeStyle": "solid",
"roughness": 1,
"opacity": 100,
"angle": 0,
"x": 754,
"y": 32,
"strokeColor": "#000000",
"backgroundColor": "transparent",
"width": 194,
"height": 24,
"seed": 315800227,
"groupIds": [],
"roundness": null,
"boundElements": [],
"updated": 1674718439209,
"link": null,
"locked": false,
"subtype": "math",
"customData": {
"useTex": true,
"mathOnly": true,
"ariaLabel": "\\mathbf{F}= q\\left(\\mathbf{E}+ \\mathbf{v}\\times \\
mathbf{B}\\right)"
},
"fontSize": 20,
"fontFamily": 2,
"text": "\\mathbf{F}= q\\left(\\mathbf{E}+ \\mathbf{v}\\times \\mathbf{B}\\
right)",
"baseline": 18,
"textAlign": "left",
"verticalAlign": "top",
"containerId": null,
"originalText": "\\mathbf{F}= q\\left(\\mathbf{E}+ \\mathbf{v}\\times \\
mathbf{B}\\right)"
}
],
"appState": {
"gridSize": null,
"viewBackgroundColor": "#ffffff"
},
"files": {}
}
```

Obviously there isn't a one-to-one mapping between a typical input document 
and what's possible or even desirable in a whiteboard context, but for the 
limited scope of producing a bunch of equations automatically, it would be 
very nice to have a pandoc writer. 

At the moment I'm using a lua filter someone kindly contributed on this 
mailing list, to simply extract all maths from an input file, 

```
_ENV = pandoc
local math_elements = List{}
return {
-- first document pass
{Math = function (m) 

local comment = "math start"
math_elements:insert(comment)
math_elements:insert(m)
end},
-- second document pass
{Pandoc = function (_) return Pandoc(math_elements:map(Plain)) end},
} 
```

I then use a custom script (I wrote it in R since I'm more fluent in it) to 
produce suitable json 
(https://github.com/baptiste/minixcali/blob/main/R/elements.R#L227)

It works, but I feel this last step should really be implemented as a 
custom Writer, especially if other people want to try this; unfortunately, 
my lua skills are currently very limited. Any pointers / starter / 
suggestions would be much appreciated!

Best regards,

baptiste



*: fantastic work by user DanielJGeiger, 
https://github.com/excalidraw/excalidraw/pull/6037

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