Re: [ft-devel] [GSoC] Initial canvas results

[suzuki toshiya]

Dear Arvinder,

On 2017/06/22 0:54, Arvinder Bhathal wrote:
> First, Suzuki-san had mentioned the following regarding canvas:
>
> > And, although I've not checked the latest implementation of Canvas
> > in modern browsers, I remember some experts complained that the
> > color or grayscale levels might be changed during the loading of
> > the pixel data onto canvas element. For bi-level monochrome images,
> > it would not be harmful, but, if you would consider the subpixel
> > or anti-alias rendering, please prepare some code to check the
> > hi-fideliy of canvas implementations.
>
>
> Canvas does report incorrect pixel data in the case when individual
> pixels are set using putImageData() with the alpha channel anything
> other than 255 or 0, then read using getImageData(). This is because
> pixels are set using straight alpha channel and canvas converts it to
> pre-multiplied. This would be a problem if we want to change pixel
> data in the browser. But, I believe we'll only be reading pixel data.
> My tests confirm canvas reports the correct pixel data for grayscale
> anti-aliased images. Iirc, FT uses pre-multiplied alpha values for
> subpixel rendering and canvas should also report the correct pixel
> data. I am testing this next.

That's good to hear, thank you for confirmation.

> Next, what I've done with canvas so far:
>
> - Generate grayscale bitmaps for characters A - Z using FT 2.5.1 and 2.8*
> - Manually populate an HTML table with a few glyphs from both versions**
> - Use JS to go through an array of all images in the current pages and
> put them into a canvas element, zoomed in without smoothing
>
> * I chose such an old version of FT to guarantee rendering differences
> ** Something to note is canvas requires a URL to the image or an HTML
> element with the image. So, the HTML generation code needs to create
> <img> elements of all the glyphs with the appropriate filename/type,
> which are then stored in canvas elements when the .html file is
> opened.
>
> For the browser, I see the following as next steps:
> - Confirm pixel data for subpixel rendering
> - Test direct HTML generation using C or with an HTML templating
> library to ensure no surprises later on
> These two I will report back on tonight.

It should be interesting.

> Regarding the glyph comparison, almost every pair of glyphs from 2.5.1
> and 2.8 differ in their dimensions. As Hin-Tak has mentioned before,
> 1-to-1 pixel comparisons are pretty much out. I've yet to try using
> the surrounding 4 or 8 pixels.
>
> My code is in the arv-test branch and I put the files inside the
> glyphtest folder. The bitmap generation code is a small snippet I
> wrote using the QDBMP library. Of course, Kushal's code will be used
> later on.

So now your code is ready to build & execute? I will take a look...

> For now, I will work on the two items I listed above. Please respond
> with any comments or concerns.

If I have something in the execution of your code, I will ask
something, but I don't have at present. Please proceed.

Regards,
mpsuzuki

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