This PR changes the way we store bounding boxes so that they use less
memory and can be more easily shared across threads in the future.
Instead of storing the bounding box and list of dependencies for each
operation that renders _something_, we now only store the bounding box
of _every_ operation and no dependencies list. The bounding box of
each operation covers the bounding box of all the operations affected
by it that render something. For example, the bounding box of a
`setFont` operation will be the bounding box of all the `showText`
operations that use that font.
This affects the debugging experience in pdfBug, since now the bounding
box of an operation may be larger than what it renders itself. To help
with this, now when hovering on an operation we also highlight (in red)
all its dependents. We highlight with white stripes operations that do
not affect any part of the page (i.e. with an empty bbox).
To save memory, we now save bounding box x/y coordinates as uint8
rather than float64. This effectively gives us a 256x256 uniform grid
that covers the page, which is high enough resolution for the usecase.
This commit is a first step towards #6419, and it can also help with
first compute which ops can affect what is visible in that part of
the page.
This commit adds logic to track operations with their respective
bounding boxes. Only operations that actually cause something to
be rendered have a bounding box and dependencies.
Consider the following example:
```
0. setFillRGBColor
1. beginText
2. showText "Hello"
3. endText
4. constructPath [...] -> eoFill
```
here we have three rendering operations: the showText op (2) and the
path (4). (2) depends on (0), (1) and (3), while (4) only depends on
(0). Both (2) and (4) have a bounding box.
This tracking happens when first rendering a PDF: we then use the
recorded information to optimize future partial renderings of a PDF, so
that we can skip operations that do not affected the PDF area on the
canvas.
All this logic only runs when the new `enableOptimizedPartialRendering`
preference, disabled by default, is enabled.
The bounding boxes and dependencies are also shown in the pdfBug
stepper. When hovering over a step now:
- it highlights the steps that they depend on
- it highlights on the PDF itself the bounding box
This is a precursor to moving the call into a
worker thread to let us use `OffscreenCanvas`. The
current position wouldn't work since we make
transformations to the canvas object after the
getContext call, which isn't allowed for
OffscreenCanvas. Also it isn't allowed to clone or
`transferControlToOffscreen` the canvas after the
`getContext` call.
Instead, we update the visible canvas every 500ms.
With large canvas, updating at 60fps lead to a lot gfx transactions and it can take a lot of time.
For example, with wuppertal_2012.pdf on Windows, displaying it at 150% takes around 14 min !!! without
this patch when it takes only around 14 sec with. Even at 30% it helps to improve the performance
by around 20%.
This restores the behaviour that existed prior to PR 19128, see e.g. 8727a04ae5/web/pdf_page_view.js (L908-L911), since `RenderingCancelledException` should still overwrite any previously seen Error.
When rendering big PDF pages at high zoom levels, we currently fall back
to CSS zoom to avoid rendering canvases with too many pixels. This
causes zoomed in PDF to look blurry, and the text to be potentially
unreadable.
This commit adds support for rendering _part_ of a page (called
`PDFPageDetailView` in the code), so that we can render portion of a
page in a smaller canvas without hiting the maximun canvas size limit.
Specifically, we render an area of that page that is slightly larger
than the area that is visible on the screen (100% larger in each
direction, unless we have to limit it due to the maximum canvas size).
As the user scrolls around the page, we re-render a new area centered
around what is currently visible.
This base class contains the generic logic for:
- Creating a canvas and showing when appropriate
- Rendering in the canvas
- Keeping track of the rendering state
