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Re: [Public WebGL] Re: ReadPixels
- To: Gregg Tavares <firstname.lastname@example.org>
- Subject: Re: [Public WebGL] Re: ReadPixels
- From: Kenneth Russell <email@example.com>
- Date: Thu, 1 Apr 2010 15:51:45 -0700
- Cc: public webgl <firstname.lastname@example.org>
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On Wed, Mar 24, 2010 at 1:07 AM, Gregg Tavares <email@example.com> wrote:
> On Fri, Mar 19, 2010 at 10:30 AM, Gregg Tavares <firstname.lastname@example.org> wrote:
>> A couple of readPixels issues
>> 1) I noticed today that apparently some drivers require a glFlush before
>> calling glReadPixels.
>> That's seem like a bug in the driver. The question though is do we want to
>> do or say anything in the spec about this? Like a recommendation that an
>> implementation call glFlush (or glFinish?) before calling glReadPixels just
>> so we don't get inconsistent results across systems?
>> 2) the WebGL readPixels spec doesn't say what the effect of
>> pixelStorei(gl.PACK_ALIGNMENT, ...) has on the buffer returned.
> No comments by anyone on this?
Sorry, I was swamped for the last couple of weeks.
PACK_ALIGNMENT should implicitly change the size of the returned
buffer. In the WebKit WebGL implementation there are a lot of cases
currently unimplemented (basically only RGBA readbacks with
PACK_ALIGNMENT <= 4 are correct) but basically it should honor both
the pack alignment and the requested format. There's no other reason
to support PACK_ALIGNMENT as an argument to pixelStorei.
The last row should not be padded. Row starts are rounded up to the
>> Does the buffer returned have rows padded to the current PACK_ALIGNMENT
>> If so, is the last row of the buffer padded?
>> In otherwords,
>> gl.pixelStorei(gl.PACK_ALIGNMENT, 4);
>> var w = 5;
>> var h = 3;
>> var buf = gl.readPixels(0, 0, w, h, gl.RGB, gl.UNSIGNED_BYTE);
>> var bytesPerPixel = 3; // RGB
>> buf.length == w * bytesPerPixel * h ? (each row is 15 bytes)
>> buf.length == (w * bytesPerPixel + 1) * h ? (each row is 16 bytes,
>> buf.length == (w * bytesPerPixel + 1) * (h - 1) + (w * bytesPerPixel) ?
>> (all but the last row is padded)
>> Off the top of my head I feel like since unlike C the user is not passing
>> in the buffer then I'd prefer the rows to be unpadded always but maybe
>> someone else has a good reason to keep them padded?
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