The rendering pipeline¶

Explanation: how a page becomes bytes on the wire, one stage at a time. Each stage is a pure function over values, which is why nearly all of it runs without a BBC Micro.

Document                                 what is to be said
   │  sextile.content.blocks
   │  sextile.viewdata.typesetting
   ▼
Row  (text, colour, indent)
   │  sextile.formatting, sextile.layout
   │  sextile.viewdata.canvas
   ▼
Frame  (24 × 40 cells)                    what the screen shows
   │  sextile.viewdata.frame
   ▼
bytes                                     the wire stream
   │  sextile.viewdata.encoding

A Document is structural, not typographic: Paragraph, Quote, Code and the rest say what a thing is, and colour is spent telling a quotation from a listing rather than on an italic. typesetting.rows_for flattens it into Rows — a quotation cyan, a listing green, nesting indented — and leaves where a frame ends to the layer that draws them.

A document, drawn¶

from sextile import Page, PageLayout, PageRequest, PageRouter, Sextile
from sextile.content.blocks import Code, Document, Paragraph
from sextile.formatting import Prose
from sextile.viewdata.typesetting import rows_for

_DOC = Document(blocks=(
    Paragraph(("A page begins as a document.",)),
    Code(("PRINT CHR$(141)",)),
))

router = PageRouter()


@router.page("1", name="page", title="Pipeline")
async def page(request: PageRequest) -> Page:
    return PageLayout(title="PIPELINE", parts=[Prose(entries=rows_for(_DOC))]).build(request)


app = Sextile(name="Demo", pages=[*router])

 PIPELINE                             1a
  
A page begins as a document.            
                                        
 PRINT CHR$(141)                        
                                        
                                        
                                        
                                        
                                        
                                        
                                        
                                        
                                        
                                        
                                        
                                        
                                        
                                        
                                        
                                        
                                        
  
 0 index

The same frame, on the wire¶

viewdata.frame is a fixed grid, not a stream of writes, and serialising it is: hide the cursor, clear, home, then the cells that say something. The C0 range holds two namespaces — a bare byte is screen or cursor control, ESC then byte

0x40 a teletext attribute — kept straight in viewdata.encoding and measured in Viewdata encoding. The frame above goes down the wire as:

14 0C 1E 1B 46 50 49 50 45 4C 49 4E 45 20 20 20  |....FPIPELINE   |
20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20  |                |
20 20 20 20 20 20 20 20 20 1B 47 31 61 1B 5A 1B  |         .G1a.Z.|
54 7F 7F 7F 7F 7F 7F 7F 7F 7F 7F 7F 7F 7F 7F 7F  |T...............|
7F 7F 7F 7F 7F 7F 7F 7F 7F 7F 7F 7F 7F 7F 7F 7F  |................|
7F 7F 7F 7F 7F 0D 0A 41 20 70 61 67 65 20 62 65  |.......A page be|
67 69 6E 73 20 61 73 20 61 20 64 6F 63 75 6D 65  |gins as a docume|
6E 74 2E 0D 0A 0D 0A 1B 42 50 52 49 4E 54 20 43  |nt......BPRINT C|
48 52 24 28 31 34 31 29 0D 0A 0D 0A 0D 0A 0D 0A  |HR$(141)........|
0D 0A 0D 0A 0D 0A 0D 0A 0D 0A 0D 0A 0D 0A 0D 0A  |................|
00A0  0D 0A 0D 0A 0D 0A 0D 0A 0D 0A 0D 0A 1B 5A 1B 54  |.............Z.T|
00B0  7F 7F 7F 7F 7F 7F 7F 7F 7F 7F 7F 7F 7F 7F 7F 7F  |................|
00C0  7F 7F 7F 7F 7F 7F 7F 7F 7F 7F 7F 7F 7F 7F 7F 7F  |................|
00D0  7F 7F 7F 7F 0D 0A 1B 43 30 20 69 6E 64 65 78     |.......C0 index|

Economy on the wire¶

Two measures cut a frame’s bytes without changing a pixel, both settled on a real screen and written up in Viewdata encoding:

A colour attribute occupies a cell, and canvas counts it, so a row that changes colour twice has thirty-eight columns for text and no layer above has to know. It is why colour could not have been added later.
Trailing blanks are not sent. The frame clears the screen first, so a space at the end of a row overwrites nothing; after the last row with anything on it, nothing is sent at all. Real pages lose between a third and three quarters of their bytes.