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

[Tristan Schmelcher]

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.

>
> 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
>