[PD] [Gem] Modifying single pixel of pix image

[cyrille henry]

you can use sigle buffer rendering.
see example 04.pix/10.pixDataSimple.pd
cyrille


Le 09/04/2017 à 09:23, Roman Haefeli a écrit :
> Thank you, Christof and Cyrille, for your insights. I think I
> understood the concept. But do I need even framebuffer ping-ponging
> when I only _add_ to the framebuffer? Let's say I add more and more
> pixel-sized white squares to the buffer, I can use this as my mask, no?
>
> Roman
>
> On Sam, 2017-04-08 at 11:05 +0200, cyrille henry wrote:
>> hello,
>> I use ping "pong buffer" frequentlly : using 2 framebuffer is a good
>> way to go.
>>
>> There is an exemple in 10.glsl/07.framebuffer_and_shader : I use a
>> "ping pong" to create the wave physical model.
>>
>> For your application, you don't even need a shader to alter the image
>> : you can draw the image, then draw a pixel size square where you
>> want to modify it.
>>
>> c
>>
>>
>> Le 08/04/2017 à 10:51, Christof Ressi a écrit :
>>>
>>> You could also experiment with fragment shader + "ping pong"
>>> framebuffers:
>>>
>>> shaders usually don't have 'memory' of the last frame(s), so the
>>> idea is that you have two framebuffers and *alternately* take one's
>>> texture as input and draw to the other framebuffer. this way you
>>> can work on the previous frame and manipulate the alpha channel,
>>> e.g. subtract values from your alpha pixels based on some random
>>> function etc. You can define some uniforms to control the
>>> parameters of your decay formula from outside.
>>>
>>> Framebuffer ping ponging is easy in openFrameworks, but I have
>>> never done it in GEM (honestly I haven't used GEM for a long time
>>> now). I guess you can use two [gemframebuffer] objects and switch
>>> your connections every other frame...
>>>
>>>
>>>>
>>>> Gesendet: Samstag, 08. April 2017 um 10:32 Uhr
>>>> Von: "Christof Ressi" <christof.ressi at gmx.at>
>>>> An: "Roman Haefeli" <reduzent at gmail.com>
>>>> Cc: pd-list <pd-list at iem.at>
>>>> Betreff: Re: [PD] [Gem] Modifying single pixel of pix image
>>>>
>>>> That's funny, I just did exactly this recently, but in
>>>> openFrameworks. If you want to do it entirely in GL (which will
>>>> be fastest), this is how I did it:
>>>>
>>>> start with a white framebuffer and gradually draw black stuff in
>>>> there. Don't clear the framebuffer so it accumulates. You can
>>>> even draw with alpha blending to make the decomposition smooth.
>>>> Then simply use the texture of the framebuffer as the alpha mask
>>>> for your image texture.
>>>>
>>>> I couldn't see how to do GL alpha masking in GEM but you can
>>>> easily write a little shader which takes the two textures as
>>>> uniforms, then you would just need to take one channel of the
>>>> mask texture and copy it to the alpha channel of your image
>>>> texture.
>>>>
>>>> You can also try on the CPU level: create your mask in a table,
>>>> make it to a greyscale image and use pix_takealpha to copy your
>>>> alpha mask to your image pixels. Of course, this will be much
>>>> slower.
>>>>
>>>> You can also mix the two approaches: create the mask on the CPU
>>>> and do actually masking in GL.
>>>>
>>>> Hope that helps!
>>>>
>>>>
>>>>>
>>>>> Gesendet: Samstag, 08. April 2017 um 10:02 Uhr
>>>>> Von: "Roman Haefeli" <reduzent at gmail.com>
>>>>> An: pd-list <pd-list at iem.at>
>>>>> Betreff: Re: [PD] [Gem] Modifying single pixel of pix image
>>>>>
>>>>> On Sam, 2017-04-08 at 08:29 +0200, Christof Ressi wrote:
>>>>>>
>>>>>> If it can be on the GPU, use a fragment shader! Do you work
>>>>>> with a
>>>>>> formula or do you set the alpha values by hand?
>>>>> By Hand (maybe later by a formula). Is it possible?
>>>>>
>>>>> The idea is to have two overlaying images. The visible one
>>>>> slowly
>>>>> decomposes over time so that the image behind appears. I
>>>>> thought about
>>>>> setting alpha to for more and more pixels as a way to decompose
>>>>> the
>>>>> front image. Maybe there is another/better way?
>>>>>
>>>>> Roman
>>>>>
>>>>>
>>>>>>
>>>>>>>
>>>>>>> Gesendet: Freitag, 07. April 2017 um 23:39 Uhr
>>>>>>> Von: "Roman Haefeli" <reduzent at gmail.com>
>>>>>>> An: pd-list at lists.iem.at
>>>>>>> Betreff: [PD] [Gem] Modifying single pixel of pix image
>>>>>>>
>>>>>>> Hi
>>>>>>>
>>>>>>> Is it possible to manipulate a single pixel of a an image
>>>>>>> loaded by
>>>>>>> [pix_image]? Specifically, I'd like to change the alpha
>>>>>>> value of
>>>>>>> certain pixels. It doesn't matter to me whether the
>>>>>>> manipulation
>>>>>>> happens in the pix realm or the GL realm. Currently I can
>>>>>>> think
>>>>>>> only of
>>>>>>> cumbersome ways like using [pix_dump] -> [pix_set] and
>>>>>>> applying the
>>>>>>> manipulation to the largish list passed between them. Or by
>>>>>>> using
>>>>>>> [pix_pix2sig~] -> [pix_sig2pix~] which is basically the
>>>>>>> same, but
>>>>>>> would
>>>>>>> allow to do the manipulation on a table.
>>>>>>>
>>>>>>> Roman
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