I'm very reluctant to expose font metrics and information (yet). I
think once you start getting into specifying fonts, you open up a can
of worms that would make this sort of API addition a lot harder.
dave
On Oct 24, 2006, at 4:05 PM, Stefan Haustein wrote:
Hi David,
I think it is very important to be able to determine the rendered
size of the text. Otherwise, there is no reliable way to make sure
things do not overlap. Also, some kinds of applications (flash-like
effects, labels expressing weight or distance, WYSIWYG text
editors) may require variable font sizes or styles.
What do you think about
context.textStyle = "barchart"; // style by reference
context.textStyle = { // set style directly
"font-size": "8px",
"font-family": "Monaco, monospace"
}
context.drawText(x,y,string); context.getFontAscent();
context.getFontDescent();
context.getFontLeading();
context.getTextWidth(string);
Best regards,
Stefan
David Hyatt wrote:
I think a drawText method would be extremely useful.
Rather than specifying stylistic information explicitly (via a
font object), I'd use a special parenthetical pseudo-element. thus
allowing the author to specify the style as for any other element
on a page.... something like this...
canvas::canvas-text(barchart)
{
font-size: 8px;
font-family: Monaco, monospace;
}
and then the API would be something like:
drawText(y-coord of baseline, "barchart", myText)
and letter-spacing/word-spacing would work, small-caps would work,
text-shadow would work, etc. etc.
fitTextToPath might be an interesting API too.
dave
([EMAIL PROTECTED])
On Oct 23, 2006, at 4:07 PM, Stefan Haustein wrote:
Gervase Markham wrote:
Stefan Haustein wrote:
I think drawElement(elem) opens up a whole new can of worms:
- how would an application determine the size of the text box?
- where is the baseline position, needed for exact axis label
positioning?
- there are probably issues with dynamically calculated text
values
- code with lots of cross references to elements will be
difficult to read
- it needs to be specified whether css properties are inherited
from the parent element of "elem".
- how much horizontal/vertical space is drawElement permitted
to use for rendering?
The answer to all of these things is that the browser renders
all the elements in the page as it would if the <canvas> were
not supported and the alternate content were being used. It then
basically screenshots the area corresponding to the element
(yes, I know this needs careful definition) and draws that into
the canvas.
I do not see how your statement answers any of my questions
except from the last one. You can specify some CSS constraints,
but how do you determine the actual rendering height of a text
box with a specific width? How do you determine the pixel
position of the baseline? The cross reference and the dynamic
text issues are not addressed at all.
Like I said, we want to leverage the browser's deep and complex
knowledge of text rendering as much as possible, and just take
the resulting pixel output as it would be shown to the user.
- the implementation in actual browsers may be more complex
than it seems because of problems with internal data structures
for rendering hints and implicitly introducing the ability to
render the same element twice.
- what happens with contained plugins, canvas elements, self-
references... all this stuff needs to be well-defined
Indeed. I know it's easy to state and there are edge cases. But
we could put limits on it like e.g. no plugins, no <object>, and
still have something very useful for rendering text.
So I assume we agree that the element rendering proposal would
still need significant specification work and is probably much
more difficult to implement. The element rendering approach may
make working with bulk text simpler, but this case is already
handled quite fine by HTML outside the Canvas element. By asking
for too much, we may end up with nothing at all.
Andrew has provided a clear and simple proposal that can easily
be implemented without too much consideration of side effects.
Putting labels on maps, precise text positioning, starwars-like
3d scrolling text, labels for game characters or in physics
simulations, all the stuff that could only be done in a canvas
element, is trivial to implement with the drawText() approach,
but seems much more complex or impossible with the element
rendering approach.
Moreover, drawElement() would not solve the drawText problem
for non-browser environments such as Rhino.
How are we anticipating <canvas> might be used in a non-browser
context?
Canvas and some other parts of the spec (e.g. connections) may
make a lot of sense for Javascript outside of the browser
context. This may be outside of the scope of WHATWG, but if we
can take out some building blocks and use them somewhere else,
this is at least a sign of good and modular API design.
Best regards,
Stefan