One lookup, natural_span, maps a photograph's aspect to the columns its
cell spans, and the same number names its thumbnail class, Wide2, Wide3
or Wide4, 512 pixels of long edge per column, so a 4:1 panorama is as
sharp across four columns as a frame is in one. The boundaries are
where the two neighbouring cells would leave the same share of
themselves undrawn, sqrt(s(s+1)): 2.45 and 3.46, with the first at 1.9
so a 3:2 frame stays a frame. A grid too narrow for the class falls
back to the widest that fits, the tablet gives the whole row, and a
cell asks for the class it is actually drawn at: its span, but never
more than its own class. The merge renders each wide class up to the
composite's own, which covers every fallback.
A 4:1 composite drawn in one square cell is a strip a few pixels high.
A photograph about twice as wide as it is tall (1.9 and up) now spans
two columns, three from 2.9, with a thumbnail class of its own whose
long edge is sized for that width; on the tablet, or where the columns
are too few to put it beside anything, it takes the whole row.
Rows are computed in one place, library_ui::layout. The grid is a
lattice of slots: each cell is drawn at the slot Rust gives it, and a
wide cell that would not fit in what is left of a row starts the next
one, leaving the gap empty so the grid still reads in capture order.
The scrollbar spans the slots, a scroll reports a slot that the layout
turns back into an ordinal, and scrubs, restores and the cursor go
through the same conversion. Up and down step by rows through the
layout rather than by a row's worth of ordinals; left and right, a
shift-click's run, burst folding and the timeline are ordinal-based
and unchanged.
The window's own read carries each photograph's w and h, so the cells
know their shape with no query per cell. Where the wide ones sit in
the whole list is one query, run when what the grid lists changes or
when a window finds the layout out of date, and a library with no
panorama answers it from a partial index created on first use
(images_wide), not a schema bump. The merge makes the wide thumbnail
for a wide composite along with the others.
The DNG a merge writes has no embedded preview, and an embedded preview
is all the grid's thumbnail path reads, so a composite stood in the grid
as a blank cell until it was opened. Reading an 800 MB file back to make
one would cost what the merge already has in hand.
The bands are box-reduced as they are written, after the border fill,
into a copy 4096 pixels long. That copy is written as a linear DNG in
memory with the composite's own profile, header and crop, and opened
through open_session, the function develop opens every file with: the
same decode, the default graph and view transform, the as-shot white
balance and the conversion to the display's space. The grid and large
thumbnails are rendered from that session, staged beside the payload
before the rename releases it to a drain, and put in the store under the
file id once the upload has learned it. Until then the grid draws them
from memory, so the cell is not blank while the file is uploading.
A test develops a synthetic composite both ways, the whole file as
develop opens it and the merge's reduced copy, and holds the thumbnail's
mean, 95th and 99.5th luma percentiles to within 3-4 levels of develop's
render; the naive balanced-and-gamma picture the merge's preview draws
misses by 13.
dtourolle