Coverage Sampling Anti Aliasing in Brief
What is Coverage Sampling Anti Aliasing? Well, lets look at NVIDIA's description of CSAA (taken from NVIDIA's CSAA PDF brief, here):"In summary, CSAA produces antialiased images that rival the quality of 8x or 16x MSAA, while introducing only a minimal performance hit over standard (typically 4x) MSAA. It works by introducing the concept of a new sample type: a sample that represents coverage. This differs from previous AA techniques where coverage was always inherently tied to another sample type. In supersampling for example, each sample represents shaded color, stored color/z/stencil, and coverage, which essentially amounts to rendering to an oversized buffer and downfiltering. MSAA reduces the shader overhead of this operation by decoupling shaded samples from stored color and coverage; this allows applications using antialiasing to operate with fewer shaded samples while maintaining the same quality color/z/stencil and coverage sampling. CSAA further optimizes this process by decoupling coverage from color/z/stencil, thus reducing bandwidth and storage costs."
On the GeForce 8800, the following CSAA modes are supported:
| CSAA Mode | Number of Color/Z/Stencil Samples |
Number of Coverage Samples |
| 8x | 4 | 8 |
| 8xQ (Quality) | 8 | 8 |
| 16x | 4 | 16 |
| 16xQ (Quality) | 8 | 16 |
So, 8xQ CSAA is the only 'true' 8x MSAA (in the traditional sense), using 8 color / Z / stencil samples, unlike 8x which uses 4 color / Z / stencil samples but 8 coverage samples. We have a little gripe with NVIDIA - we think it would be more 'correct' to simply use 8x for 8xQ instead and different ones for the other CSAA modes (for example 4x8 for 8x CSAA, 4x16 for 16x CSAA and 8x8 for 16xQ). Such an arrangement will likely cause less confusion for average users, who will likely assume 4x (MSAA) and 8x (CSAA) employ the same method. In our opinion, calling NVIDIA's GeForce 8800 8x or 16x CSAA as 8 or 16 samples anti aliasing, respectively, is 'bending' the truth quite a bit, some might even say its misleading. Let's see if coverage sampling anti aliasing (CSAA) offer something 'more' compared to traditional multi sample anti aliasing.
GeForce 7950GT
Radeon X1950 Pro
GeForce 8800GTS
This small application from ToMMTi-Systems shows a unique perspective on anti aliasing. With it, we can see how effective anti aliasing implementation on lines with different slopes. If only there was also support curves in addition to lines, it would just be perfect. Anyway, back to the shots. One of the easiest spot to see differences is the third quadrant of the shot - between 6 and 9 o clock (180 to 270 degrees). At 4x MSAA, the Radeon X1950 Pro and GeForce 8800 was able to display the entire array of lines, while the GeForce 7950GT seems to 'lost' two lines (or triangles if you prefer). If you look REAL close (or zoom in), you'll see the Radeon X1950 Pro shot displays less aliasing in this area, even compared to the GeForce 8800. Moving to the center, you'll also see a more 'complete' picture on both the Radeon X1950 Pro and GeForce 8800 - no loss of details here.
Now, let's focus on 8xS, 8x CSAA an 8xQ MSAA shots. The GeForce 7950GT with 8xS still 'lost' some detail compared to 8x CSAA and 8xQ MSAA on the GeForce 8800. Good or bad, well that depends. Look at the whole picture and you'll definitely see 8xS does a better job in reducing aliasing overall. Both approaches have disadvantages, you either lost details (8xS) or have more aliasing (8xQ and 8x CSAA). 8x CSAA and 8xQ MSAA shots are actually pretty close to each other, though 8xQ seem to offer a more 'distinctive' image - you'll notice more pixels near the center with 8xQ MSAA and 8x CSAA.
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