720p 64bit framebuffer size? Educated guesses here please

[Squeak]

How much bandwidth and RAM does a 720p x 64bit framebuffer with 4x AA take up.
My (uneducated) ballpark guess would be in the area of 5Gb/s with all bandwidth saving features on and a reasonable overdraw of 5.

 

[pc999]

Isnt 1280X720X64X4X30(or 60FPS)=7077888000b=~7Gb@30FPS

Anyway I am no programer, please correct me

@60FPS=~14gb

And 1980X1080X64X60 (Ithink they refered this in the press c.)=~8gb

 

[Apoc]

Isnt 1280X720X64X4X30(or 60FPS)=7077888000b=~7Gb@30FPS

Anyway I am no programer, please correct me

@60FPS=~14gb

And 1980X1080X64X60 (Ithink they refered this in the press c.)=~8gb

I think it should be like that:

1280 x 720 x (2bytes per component x 4 component) x 4 (AA)= 29.491.200 Bytes per frame x 60 fps = 1.769.472.000 Bytes

All of these without any bandwidth saving feature applied of course.

 

[jvd]

where u getting the 64bit part from ? shouldn't it be 32bit as that is what its renderered in ?

 

[Apoc]

where u getting the 64bit part from ? shouldn't it be 32bit as that is what its renderered in ?

Is it rendered at 32 bit? I thought it would render the framebuffer full precision and then "downsampled" it to 32 bits on output to the monitor/tv.

 

[jvd]

where u getting the 64bit part from ? shouldn't it be 32bit as that is what its renderered in ?

Is it rendered at 32 bit? I thought it would render the framebuffer full precision and then "downsampled" it to 32 bits on output to the monitor/tv.

I thought all video cards rendered in 32 bits , well i guess 28bits and 4 bits for alpha or whatever ?

Or is that color percision ?

 

[Apoc]

where u getting the 64bit part from ? shouldn't it be 32bit as that is what its renderered in ?

Is it rendered at 32 bit? I thought it would render the framebuffer full precision and then "downsampled" it to 32 bits on output to the monitor/tv.

I thought all video cards rendered in 32 bits , well i guess 28bits and 4 bits for alpha or whatever ?

Or is that color percision ?

With the nv40 i THINK they introduced the 64 bit framebuffer, that's why it takes up LOTS of bandwidth. And i think they couldn't render HDR properly if they kept rendering at the same precision. I guess it's 64 bit internal framebuffer precision and 32 bits on output via tone mapping done on the ramdac.

 

[Titanio]

I've always wondered if for 4xMSAA you just multiply the bandwidth figure by 4..?

And yes, I think it should be a 32-bit framebuffer.

Assuming these are the case, then @ 60fps with 5x overdraw @ 720p with 4xMSAA, your bandwidth requirement would be less than 4.5GB/s.

I somehow wonder about the figure for antialiasing however..is it really just a simple multiply? Is it even static?

edit - Apoc's explanation re. 32-bit vs 64-bit makes sense. So I guess you'd double the figure.

edit 2 - HDR, if used, also has to be taken into account - anyone have any ideas how it affects bandwidth requirements?

 

[Shifty Geezer]

AA should be a simple multiply, as its n times the samples per pixel. eg. 4x MSAA adds four pixels around the target pixel for sampling, so requires 5x thebandwidth of just that pixel alone without AA.

 

[Titanio]

AA should be a simple multiply, as its n times the samples per pixel. eg. 4x MSAA adds four pixels around the target pixel for sampling, so requires 5x thebandwidth of just that pixel alone without AA.

Cheers! So..64-bit framebuffer, 720p, 5x overdraw, 4xMSAA, 60fps = 11GB/s? And of course, that's without any bandwidth-saving techniques.

At least we're starting to get somewhere!

So HDR...any guesses?

 

[jvd]

wouldn't mind knowing the fp10 , fp16 and fp32 hits . That would be interesting to see .

 

[Thowllly]

AA should be a simple multiply, as its n times the samples per pixel. eg. 4x MSAA adds four pixels around the target pixel for sampling, so requires 5x thebandwidth of just that pixel alone without AA.

No. 4xAA does not add 4 samples around a central one, it replaces the central one with 4 samples. Maybe you've been confused by those MSAA sample pattern screenshots? Also, MS is really easy to compress, and texture and vertex BW does not increase, so the BW requirement is much less than 4x the no AA requirement. Best case BW requirement for 4xMSAA is: same as no AA. Worst case: 4x the BW of no AA. Real world: somewhere in between.

 

[Shifty Geezer]

fp 10's the same as 32 bit colour (32 bits per pixel). fp 16's double that (assuming 16 bit alpha) at 64 bits per pixel.

 

[Titanio]

fp 10's the same as 32 bit colour (32 bits per pixel). fp 16's double that (assuming 16 bit alpha) at 64 bits per pixel.

So if we're already using a 64-bit framebuffer, fp16 is no different?

fp32 is double fp16, basically a 128-bit framebuffer?

so:

720p, 5x overdraw, 60fps, 4xMSAA, fp10 HDR = ~4.4GB/s (assuming what Thowllly says is correct about the 4 MSAA samples replacing the one sample)

720p, 5x overdraw, 60fps, 4xMSAA, fp16 HDR = ~8.8GB/s

720p, 5x overdraw, 60fps, 4xMSAA, fp32 HDR = ~17.6GB/s

Anyone disagree?

Of course, in all instances this isn't accounting for bandwidth saving techniques.

 

[jvd]

looking through daves preview of the nv40 it doesn't seem to have a 64bit framebuffer.

 

[DeanoC]

looking through daves preview of the nv40 it doesn't seem to have a 64bit framebuffer.

If by framebuffer you mean the frontbuffer you can actually send to the RAMDAC then no it doesn't. However it does have 64bit and 128bit backbuffers which are also known as framebuffers...

 

[Rockster]

Are you guys ignoring the Z-buffer on purpose? Seems illogical since you can't have one without the other. That would essentially double your requirements sans compression.

 

[Titanio]

Are you guys ignoring the Z-buffer on purpose? Seems illogical since you can't have one without the other. That would essentially double your requirements sans compression.

Well one might make the distinction between framebuffer and zbuffer which is what the original poster asked about.

I'm not sure if it'd double your requirements in the above cases either..in the first perhaps (with a 32bit framebuffer), but in the second and third (64-bit and 128-bit framebuffers)? Does a zbuffer require precision beyond 32-bit? I don't think you'd be anti-aliasing your framebuffer either

 

[EasyRaider]

What does this discussion have to do with consoles? Do you think these simplistic calculations will give you any useful idea of realistic resolution/AA/backbuffer precision for PS3 or something?

At least try to account for depth buffer and compression, then maybe you would be getting somewhere, though it would still be a guess, and not a very educated one from the looks of it (no offence )

 

[EasyRaider]

I don't think you'd be anti-aliasing your framebuffer either

Uh, what else would you be anti-aliasing? Texture samples only?