[cairo] Creating an opaque fade-in effect with Cairo

[Andrea Canciani]

On Fri, Jan 21, 2011 at 1:59 AM, Tristan Schmelcher
<tschmelcher at google.com> wrote:
> On 20 January 2011 14:26, Andrea Canciani <ranma42 at gmail.com> wrote:
>> On Thu, Jan 20, 2011 at 10:54 PM, Tristan Schmelcher
>> <tschmelcher at google.com> wrote:
>>> Hello,
>>>
>>> I am developing a Cairo-based application and I'd like to fade-in an
>>> opaque image on top of a background. i.e., I want the image to start
>>> out as opaque black and gradually fade-in to its full brightness. But
>>> I can't figure out how to efficiently paint my image surface to the
>>> screen and attenuate the RGB channels at the same time. Ideally I'm
>>> looking for an API that lets me paint my source surface (the image)
>>> and takes a fractional brightness parameter in the range [0, 1] to
>>> multiply with each of the R, G, and B channels. So far the best that I
>>> have come up with is to first paint with CAIRO_OPERATOR_CLEAR and then
>>> paint with CAIRO_OPERATOR_OVER and cairo_mask(), but that seems like a
>>> lot of computational work. Is there a more efficient way to do this
>>> with Cairo?
>>
>> If your destination is always opaque, you can do everything like this:
>>
>> cairo_set_operator (cr, CAIRO_OPERATOR_IN);
>> cairo_set_source (cr, the_fading_surface);
>> cairo_paint_with_alpha (cr, the_fade_parameter);
>>
>> This should be sufficient to completely replace the old content assuming that
>> the destination has a content of type CAIRO_CONTENT_COLOR.
>> It is slightly less flexible than your code, because this will only allow
>> fading from/to black, but I expect it to be faster, because it performs only
>> one compositing operation.
>
> Thanks for the info! My destination is indeed of type
> CAIRO_CONTENT_COLOR, so your suggestion works great, but surprisingly
> it seems to be slightly slower. When I make it render as fast as it
> can, the CLEAR+OVER approach gets an average of 13 fps while the IN
> approach gets an average of 12 fps. Rather strange ... I definitely
> agree with you that it ought to be the faster method.

This probably means that with CLEAR+OVER you were hitting a fast path
in pixman, but with IN you are going through a slow path.

Profiling should tell you if this is what happens.
A trace would make it possible to cairo/pixman developers to try and reproduce
the problem. See http://www.cairographics.org/FAQ/#profiling for reference.

>
>>
>> You should obviously profile your application to check if this improves
>> its performance or not and you might also want to check what are the hot
>> functions, so that they might possibly be improved.
>>
>> Andrea
>>
>