[whatwg] Canvas lack of drawString method

Joao Eiras joao.eiras at gmail.com
Thu Oct 26 07:15:52 PDT 2006

I think mixing CSS and Canvas is a bad idea.
You could develop an application which would implement the canvas APi, and  
be easily programed with ecmascript or java.
Adding this kind of feature would force the implementor to support CSS  
too, at least partially.

Stefan Haustein <sh at kobjects.org> escreveu:

> 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
>> (hyatt at apple.com)
>> 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

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