I did a bunch of testing of various ansi escape codes (including
italics) with all three char atlas implementations (none, static,
dynamic), with and without `drawBoldTextInBrightColors`, and with and
without `enableBold` to test all of the edge cases that I could.
It turns out that `glyph.char.charCodeAt(0)` isn't equivalent to the
character's actual code because of utf-16 garbage, so this adds
`glyph.code` to the `glyph` object so that we can use the `code` value,
and don't need to regenerate it.
- We need to make a clone of this array, because ColorManager mutates
it.
- We only want to compare the first 16 colors instead of all 256, since
only the first 16 will ever change. In DynamicCharAtlas, we can pull
the additional colors from DEFAULT_ANSI_COLORS.
I tested this out by loading the demo and changing the theme a few
times.
This limits the amount of damage a pathological program can cause in
thrashing the LRU cache.
I ran the script described here:
https://github.com/xtermjs/xterm.js/pull/1327#issuecomment-374003960
Without this patch, the entire output would get drawn in one frame that
took about 140 ms to draw. With this patch, it takes about 60 ms. It's
still nowhere as good as the static or none atlas implementations, but
it's not terrible, like it was.
This should have minimal impact on real-world applications: it'll just
take a little longer for the atlas to warm up.
The implementation is a little hacky, since it involves throwing some
code into TextRenderLayer that violates the separation of concerns, but
we can clean this up later.
Pros:
- The code is simpler.
- AA rendering is consistent between cached and uncached characters.
- No long requires parsing color values to determine the alpha, which
means that we no longer depend on #rrggbbaa color value support.
Cons:
- We're drawing without SPAA in some cases where we could be.
Addresses this comment:
https://github.com/xtermjs/xterm.js/pull/1327#discussion_r176109332
Drawing to a ctx with `{alpha: true}` causes the canvas API (on chrome
at least) to switch to grayscale AA, instead of RGB subpixel AA.
This fixes it by drawing to a ctx that's using `{alpha: false}` when
posisble, and switching to a ctx that's using `{alpha: true}` only when
we have to.
There were two bugs:
- We need to clear the background of _tmpCtx instead of just drawing on
top if the background color is transparent.
- We should set the background to fully transparent if it's partially
transparent, to avoid drawing the transparent background twice.
LRUMap implements the parts of an ordered map that we need to
efficiently implement DynamicCharAtlas. It's more code, but there's a
few advantages of this approach:
- Map isn't available on some older browsers, so this removes the need
for a polyfill.
- Moving an item to the end of the map's iteration order now only
requires unlinking and linking a linked-list node, whereas before we
had to delete and re-insert our value.
- Peeking at the oldest entry in the map no longer requires allocating
and destroying an iterator.
- We can preallocate the linked-list nodes we want to improve cache
locality. Similarly, we can recycle linked-list nodes to reduce
allocations and the GC pauses those allocations may cause.
- LRUMap seems to give slightly better results in Chrome's profiler than
Map did. We now spend about 5% of our time on map operations instead
of about 10%.
- In my (limited) testing, it doesn't look like LRUMap is slowing down
over time. Map appeared to get slightly slower the longer I ran the
terminal for, either due to memory fragmentation or some sort of leak.
I still need to write some tests for LRUMap, but I've been using this
implementation for the last hour without problems.
When we remove the background from a glyph, we can keep track of if we
made all the pixels fully transparent. If we did, we can just skip
future calls to _drawFromCache, since it would be a noop anyways.
This adds a very minimal implementation of DynamicCharAtlas using a LRU
cache.
I've got some optimizations that I'll add on top of this, but this
proves the concept.
Daniel Imms
Jeff Principe
Benjamin Woodruff