Re: [PyOpenGL-Users] Rolling spectrogram with pyopengl

[Timothée L. <tim...@lp...>]

Gijs a écrit :
> On 7-12-2009 9:13, Timothée Lecomte wrote:
>>
>> Le 7 déc. 09 à 00:00, Greg Ewing a écrit :
>>
>>> Timothée Lecomte wrote:
>>>> As far as  alternatives to pure glTexSubImage2D, I have considered
>>>> and tried to  use PBO, but it does not decrease the time needed by
>>>> the two  glTexSubImage2D calls.
>>>
>>> This is puzzling, because transferring 1000 pixels of data
>>> shouldn't take very long, even if you're not doing it the
>>> most efficient way.
>>>
>>> The only thing I can think of is to try using different
>>> pixel formats for the texture data. Fastest CPU->GPU transfers
>>> occur when you use a pixel format that matches what the
>>> GPU uses, so that no conversions are needed.
>>>
>>
>> That's an advice I've seen very often on opengl pages on the web. Is
>> there a way to know at runtime what is the native format ?
>>
>> Thanks Greg.
>>
>> Timothée
>>
> Hello Timothée,
>
> I don't know of any method to determining at runtime which texture 
> transfers the fastest, except benchmarking it and see which performs 
> the best. However, most of the time GL_BGRA with 
> GL_UNSIGNED_INT_8_8_8_8 is usually the fastest. If you want to check 
> to make sure, run the code I supplied with this email. It transfers a 
> whole bunch of different formats and displays which are the fasted 
> pixel-wise and megabyte-wise.
>
> Regards, Gijs

Hello Gijs,

Thanks for your help. I found that on my machine, the best pixel rate is 
achieved with GL_BGRA and GL_UNSIGNED_INT_8_8_8_8_REV. I get 397.253 
Mp/s. Does that look ok (it's NVidia Quadro NVS 290/PCI/SSE2, on ubuntu 
karmic) ?

I have a question now: when glTexSubImage2D is called to update ~1000 
pixels on a texture of 1Mpixels, does the whole texture gets transferred 
again from system memory to video memory ?

Best regards,

Timothée