So if the Wii U does have 32mb of eDRAM on the GPU, will it be enough to store a 720p or even 1080p image and add "free" AA or motion blur or DOF without taxing the GPU? Will the Wii U be able to render 720p games natively without tiling and AA?
Ok, first off, Let me lay the basics for this, measuring how much an image weights is pure math and it's like this:
1280x720x32 = 29491200 bits =
3.5156 MB/frame (x30fps=105.47 MB/s; x60fps=210.94 MB/s)
1920x1080x32 = 66355200 bits =
7.9102 MB/frame (x30fps=237.31 MB/s; x60fps=474.61 MB/s)
(the 32 in the end is relating to the bit depth for the image; in this case 32 bits)
But it's not just this, it's an additive process; Z-Buffer is typically 32 bits as well (but can also be reduced to 24 bits) which effectively doubles the pixels being sampled and X360's "free AA" adds 24 bits per passage; because passages come in pairs 2xMSAA and 4xMSAA on X360 costs an extra 48 or 96 bits respectively.
In short, you do this… I'll do three case test scenarios here, first off the simple no-AA solution 32+32=64 bits (frame+z-buffer), second 32+32+48=112 bits (frame+z-buffer+2xMSAA), third: 32+32+96=160 bits (frame+z-buffer+4xMSAA)
1280x720x64 =
7.0313 MB/frame (x30fps=210.94 MB/s; x60fps=421.88 MB/s)
1280x720x112 =
12.3047 MB/frame (x30fps= 369.14 MB/s; x60fps= 738.28 MB/s)
1280x720x160 =
17.5781 MB/frame (x30fps= 527.34 MB/s; x60fps= 1054.69 MB/s)
No-AA, 2xMSAA and 4x MSAA respectively.
As I'm sure you understand, you'll be doing tiling from 2xMSAA and up. (and no, in 2xMSAA even reducing the z-buffer to 24 bits would still land us on 11.5 MB/frame field and rendering the frame and z-buffer on 24 bits would fall on 10.5 MB/frame range; leaving no quick fix other than tiling and/or sub 720p). I also left the MB/s math in as a means to help establish a relation of why 60 frames had a hit on framebuffer bandwidth and could act as a bottleneck; there's no 3D game at 1080p achieving 60 frames this gen (wipeout hd dynamic framebuffer doesn't count, as it adjusts frame size; closest game to doing it is Ridge Racer 7). On top of it I'm sure you know this, but framebuffer acts as a scratchpad of sorts, so you can add even more layers/passages to whatever you're doing on top; it's not uncommon for full frame distortion effects to be applied on top for example. On a side-note, on 1280x720 Wii U could do 8xMSAA without tiling, taking roughly 28 MB per frame.
With this explained I can get on to 1080p, 1920 is:
1920x1080x64 = 132710400 bits =
15.8203 MB/frame (x30fps=474.60 MB/s; x60fps=949.22 MB/s)
1920x1080x112 = 232243200 bits =
27.6855 MB/frame (x30fps=830.57 MB/s; x60fps=1661.13 MB/s) ⬅ Wii U could do 1920x1080p with 2xMSAA without tiling.
1920x1080x160 = 331776000 bits =
39.5508 MB/frame (x30fps=1186.52 MB/s; x60fps=2373.05 MB/s)
Again, No-AA, 2xMSAA and 4x MSAA respectively.
A 40 MB framebuffer seems tempting (albeit a little overkill on a platform we haven't seen pulling 1080p yet), but still the 32 MB of eDRAM on Wii-U seem very generous, should be way more straightforward than X360's eDRAM to work with; of course this is just a part of the puzzle, the rest depends on this part bandwidth and clock speed as well as console fillrate; probably why we aren't seeing more (have we seen any?) 1080p games too; still, should be way more easy to pull on it than on this gen, but I suspect most devs will prefer to spice up the graphics and/or throw in more effects/anti aliasing.
Also there's MLAA now which could spare the framebuffer a little but would erode the available fillrate (and a few stream processors along the way) so time will tell how often it'll get used on the platform with less GFlops this upcoming gen.
I hope it shed some light into the matter.