Hi Josh,
Bokeh is definitely based on the HTML5 canvas, which as you
note, generates a rasterized bitmap image. The reason you
don’t see artifacts when resizing the plot is because the
resize is not simply acting on the bitmapped image. Rather,
BokehJS has knowledge of data space bounds and screen space
bounds and uses this information to compute screen
coordinates for all actors in the scene. Whenever either of
the bounds change, the coordinates are all re-computed, and
the scene is completely redrawn (i.e., re-rasterized by the
canvas raster engine). The resize tool is actually very
simple, it just changes the canvas size, which triggers all
of the above to happen.
An interesting thing to note, however, is that the canvas
API is basically a vector API. That is, you define paths,
apply affine transformations, and then stroke/fill them. The
rasterization is largely hidden from canvas users.
Regarding the future:
* we are looking at node-canvas to provide static SVG
output. This is an often requested feature, and the only
reason it would be remotely possible without creating an
entirely new backend from scratch is because, as mentioned
above, the canvas API is a vector API. The standard canvas
output is a bitmap, but node-canvas can redirect and render
to SVG instead (apparently, and hopefully).
* we are also looking at webgl-canvas for the future as
well. Better performance in the “hundreds of thousands” of
points range is also desired, and perhaps some measure of 3D
plots as well. Please note this is extremely preliminary
and tentative, no work or even real planning has started in
this direction.
Thanks,
Bryan
> On Dec 15, 2014, at 11:40 PM, Josh Wasserstein <[email protected] >
wrote:
>
> Hi there,
>
> Prompted by the new release of Bokeh 0.7 I am
giving a talk at my company on Bokeh. I read on the main
website that most of the magic is done by BokehJS, which
exposes the control of the HTML5 Canvas. When I go to
Wikipedia, I read that “unlike canvas, which is
raster-based, SVG is vector-based, i.e., each drawn
shape is remembered as an object in a scene graph or
Document Object Model, which is subsequently rendered to
a bitmap.”
>
> This is news to me, since I thought that like Bokeh
used vector graphics under the hood (IIRC D3.js uses
SVG). Moreover, Bokeh allows you to easily do infinitely
resize and I never see the bitmap artifacts when doing
so.
>
> I know that Bokeh may end up using SVG at some
point, but for now, is it correct to say that Bokeh uses
raster-based graphics? Vector-based? Or a combination of
both?
>
> Josh
>
