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Speeding up readpixelfast?

Blitz3D Forums/Blitz3D Programming/Speeding up readpixelfast?

Pongo(Posted 2006) [#1]
Are there any tricks that can be used to speed up a readpixelfast command?

In this instance, I do not need the exact value, since I am only trying to find out if it is higher or lower than a threshold value.


jfk EO-11110(Posted 2006) [#2]
Backbuffer, Frontbuffer, Imagebuffers will differ in access speed. Just like Texturebuffers, where using Texture Flag 256 makes it much faster.

Continously reading and writing pixels is also bad, you rather should read all pixels into an array or bank, then compute and writepixelfast the whole array (has something to do with cache optimation on the CPU level).


Sir Gak(Posted 2006) [#3]
Remember also to use the LockBuffer and UnlockBuffer commands, before and after your readpixelfast commands.


jfk EO-11110(Posted 2006) [#4]
BTW now this is really confusing me, I made a test to compare the speed of readpixelfast and writepixelfast in images and textures. Now it turned out using the flag 256 is making textures slower, where it should be faster.

This is really strange. Maybe it has something to do with the quality settings of the Ati card that I have altered recently.

I do however still recommended to test the speed of the flag 256 and not to use it untested.


Ross C(Posted 2006) [#5]
Yeah, it's varied amoungst cards. It offers a large speed up here. Amount of VRAM is also important. If your vrAM doesn't have alot of capacity in the first place, forcing a texture/image into it, will cause lots of swapping.


Dreamora(Posted 2006) [#6]
Its slower on my 9700 as well if I use VRAM ... looks like it is mostly faster on system RAM based cards and old cards that were that slow and low memory that they normally were using the systemram through AGP RAM extension.


big10p(Posted 2006) [#7]
Flag 256 is slower on my (ancient) GeForce256, but faster on my (semi-ancient) GeForce2 GTS. Both cards have 32MB VRAM.


Sir Gak(Posted 2006) [#8]
I guess the confusing results on differing video graphics cards, makes it imperative that we have in our programs a set/unset screen where features can be enabled or disabled, such that the user can implement corrective changes. Otherwise, how can we as programmers guess which card or graphics environment we're sending our programs into?


big10p(Posted 2006) [#9]
Otherwise, how can we as programmers guess which card or graphics environment we're sending our programs into?
Aye, there's the rub! :)


jfk EO-11110(Posted 2006) [#10]
you can guess it when you measure the time of both, that's pretty simple. something like:

t=createtexture(512,512)
setbuffer texturebuffer(t)
t1=millisecs()
lockbuffer()
for i=0 to 100000
rgb=readpixelfast(rand(0,511),rand(0,511))
writepixelfast rand(0,511),rand(0,511),rand(0,$FFFFFF)
next
unlockbuffer()
t2=millisecs()
time1=t2-t1
freetexture t

t=createtexture(512,512,256)
setbuffer texturebuffer(t)
t1=millisecs()
lockbuffer()
for i=0 to 100000
rgb=readpixelfast(rand(0,511),rand(0,511))
writepixelfast rand(0,511),rand(0,511),rand(0,$FFFFFF)
next
unlockbuffer()
t2=millisecs()
time2=t2-t1
freetexture t

if time2<time1 then 
 print "flag 256 is faster"
else
 print "flag 256 is slower"
endif


The difrence between them is about 70% on my machine. So an image takes 130 ms, a texture with the 256 flag as well, without the flag only about 40ms.

I am still confused since I think I clearly remember the flag 256 once used to speed pixel access up massively on exactly THIS machine.


Shifty Geezer(Posted 2006) [#11]
Maybe that was speed-up in a different area? Try writing (Text) to the textures in VRAM and not and see if that speed difference matches the copypixel situation or not?

Well I've just tried that, and it makes no odds. Not in VRAM on my machine is consistently slower, but only by a few % at best, and sometimes all of 0.1% slower. There is a large discrepency between full-screen and windowed though. In windowed sometime buffers not in VRAM were faster than those in VRAM, and sometimes not.


Dreamora(Posted 2006) [#12]
Depending on the card, drawing text to a VRAM texture will totally fail.
Never count on anything beside lockbuffer with the pixel commands to work on VRAM textures.


jfk EO-11110(Posted 2006) [#13]
I only remember I made this realtime shadow demo for the code archives and after Flag 256 was added, a lot of people reported speed increase of about 70%. Maybe it's Copyrect to a texturebuffer that was used in the demo.