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

[Tristan Schmelcher]

On 21 January 2011 00:26, Andrea Canciani <ranma42 at gmail.com> wrote:
> 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.

I'll try that. Thanks. :)

>
>>
>>>
>>> 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
>>>
>>
>