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Jimkm

OpenGL Need sugestion to optimize realtime bitblt->StretchBlt (scaling takes to much CPU)

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Here's my problem. I need to copy and scale the screen, ~30 times a second (live screen drawing).
I do this by first getting a handle to the screen DC:

GetDC(NULL)
Then, I create a compatible DC using CreateCompatibleDC, create a DIB using CreateDIBSection, and select the DIB into the new DC. Now I StretchBlt from the screen DC to my own DC  (blit and scale the image) .

So we have

while(1)
{
StretchBlt ... (get the screen and resize from 1440*900 to 1200*700)

}

As you probable know the scaling takes most of the CPU and time.
If i do only

while(1){bitblt()} i get ~400 fps - i.e 400 call of bitblt per second.

if i use the while(1){StretchBlt} i get only ~25 fps - i.e 25 call of StretchBlt per second.

(I need 30). also since i am doing other things as well, CPU will not be idle so this code need to be ran at least at 34 FPS.

As said, the bottleneck is not the Blitting from the screen. the bottleneck  is the scaling (i use SetStretchBltMode(hCompatible, HALFTONE)).
with COLORONCOLOR i get 60FPS but quality is not good (if there only something in the middle of HALFTONE to COLORONCOLOR  ...).

For now i do not want to use directx or open GL.

Any idea how to make it faster (i only need ~10 more FPS :))
I have an idea how to make it faster  but i do know how to do that and if it is possible.
Idea is to:
Bitblt to get the screen into buffer A.
StretchBlt to scale the image into buffer B.
Now on the next bitblt's to use SRCINVERT (XOR) to blt only the differences between the new capture and the old and to use the differences to update the scaled buffer B so no need to do more scaling.

Again just an idea but i do not think it is possible.

I would love to hear your suggestions.

I tries to use bitblt with some custom scaling functions (instead of StretchBlt) i found on the Internet or with image libs like FreeImage but results were dramatically worst than StretchBlt  (even using BOX filtering)

Again, i know that using DirectX/opengl may be faster but currently i do not have the time to learn that or deal with that - so currently i would like to use GDI/GDI+ only.

Thanks!

 

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Hi. are you running this bitblt in a loop like here.

 

while(1)
{
StretchBlt ... (get the screen and resize from 1440*900 to 1200*700)

}

 

may be you need to just update it on a wm_paint or what ever the message is.

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I do not think that there is an option to catch when wm_paint is sent to the screen itself, i.e tocatch some event that is thrown when what you see on the computer screen is changed.

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It seems that halftone blitter is really slow, couldn belive it to be so slow and i did quick testing - it shows the same result youre talking here about

abput 40 ms - what system do you have, it may depend on the system 

 

i dont know some say that in win7 it may run even worse than xp ;/ ?

(found something on this

http://vjforums.info/threads/stretchblt-can-hang-windows-by-hogging-gdi-lock.38179/ )

 

anyway normal mode blitter working ok and you could rescale this by hand code or ev trying to do some "opengl/dx blitter" with fast hardware resizin onboard - which i was not testing yet

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Hi Fir.

I tried to use custom scalling - some even with ASM byut could not get even near strechblt - with the custom scaling fucntion i got ~10 FPS ....

And as say i do not want to use opengl/dx.

 

Am am using win7.

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Hi Fir.

I tried to use custom scalling - some even with ASM byut could not get even near strechblt - with the custom scaling fucntion i got ~10 FPS ....

And as say i do not want to use opengl/dx.

 

Am am using win7.

you may show the code, we can taka a lok here and maybe come advices or conclusions will appear -i could do the test of some down of upscalling in bitmap arrays im not sure but i think it could work at 50 (100?) hz or more - not 100% sure though

 

PS i did some simle test of down and up scaling frame bitmap into some buffer then copying this to framebuffer array again -and even with this double copying i got no trouble

 

for screens size like 500p it was 4 ms (scaling forth somewhere and copying back) for screens like 1000p it was 12 ms (for such two way: scaling+copying back), so not a problem here -

 

got no idea what this halftone is doing to be so slow but this linear scaling works ok

 

Edited by fir

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First i tried this:

http://www.geisswerks.com/ryan/FAQS/resize.html

Gave me around 10 FPS only.

I then tried FreeImage lib (FreeImage_Rescale) got around 7 FPS.

None came even close to StretchBlt preformace.

 

I think the answer should be

1) To find fastest rescal (with good quality) scaling funtion/

OR AND

2)to somehow use the scaling histroy.

I.E to try to scal only the parts that changed from the last scale - but not sure how to do that ...

Maybe with SRCINVERT (XOR) - somehow.

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got no idea what this halftone is doing to be so slow but this linear scaling works ok

 

 

 

Well without halftone the image quality is not good.

Text does not look good and somehting is unreadable.

The halftone smoth the image by avraging pixels before it scall or something like that.

 

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Scaling is expensive. This is why, for instance, Fraps only offers fullscreen or halfscreen resolution when it captures the screen, so that scaling is either unnecessary or very easy. It's just too costly to handle cases where you're not scaling down to a power of two of the original size, because you have to handle filtering of multiple overlapping pixels, which also blows your cache because the resulting memory access patterns are.. suboptimal to say the least. There is one thing you can try, which drastically helped me back when I was on my old laptop and trying to capture stuff, which is to set your screen resolution to 16-bit colors.

 

Otherwise, what I would try is instead get a copy of the screen in a GPU-bound texture, using your favorite graphics API, scale it using the pixel shader (which ought to be fast enough, even with bilinear filtering) and read it back on the CPU. But this might incur significant capture latency, and might not be feasible, and you said you don't want to do this anyway, so...

 

I'm not sure what your specific needs are, but have you considered simply getting one of those screen recording devices? They don't have any system overhead and you might be able to get one to feed back into the computer to deliver the screen contents in realtime.

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Hi Bacterius

Yes i know scaling is expensive. that is why using the scaling history might help.

I just do not know how to do that. I.E to scale only the parts from the image that changed from the last scale - that way i scale one time and on next scales i only scale

the parts that were changed - but not sure if that can be done and how.

 

P.S

Just tried CxImgLib and got only 20 FPS still lower than  StretchBlt ...

 

P.S2 lowering to 16 bit did not help and in anyway it should run on all systems without making changes - that is why recording device is not a solution.

Edited by Jimkm

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