The picture above shows the end of the rendering pipeline as the 3D Core. This 3D Core renders the game scene pixel-per-pixel of each polygon. Every time a screen pixel is finished, it is dithered down to 16 bits from the internal 24/32-bit accuracy. More details about the dithering algorithm can be found here. Basically 24/32-bit info is compressed into a 16-bit format. The 16-bit dithered pixel is stored inside the Frame Buffer. In memory, you have various Frame Buffers: a front and a back buffer. The front buffer is shown on-screen while inside the back buffer the next frame is constructed (as described before). The front buffer data (containing a finished frame) is read and is upsampled using the selective filtering technique described here. The result are screen pixels with a color accuracy of maximally 22 bits. This upsampled color is translated from its digital format into an analogue signal by the RamDAC and that signal is passed on to the monitor.
In short, from 24/32 bits to 16-bit using dithering and from 16 bits dithered "back" to 22 bits using a selective filter. I hope that this little diagram makes the location of this upsampling/filtering hardware clear.
So what about screenshots ?
As you know screen capture programs capture the digital data inside the frame buffer. This data is 16 bits and not at all equal to what Voodoo3 (and 2 and 1 ) show on the screen. The problem with the screen capture is that the selective upsampling filter is missing since that filter is located behind the frame buffer. This missing post filter is essential to show the true image quality from the Voodoo boards.
So are screenshot comparisons with TNT invalid ?
I asked Gary Tarolli from 3dfx this question, he replied that: " The only way to see the true quality is to see the image on the screen. Or to use HyperSnap, we provided special code to the developers to emulate what our filtering does." This means that inside the HyperSnap program, there is a post filter similar to the filter that is implemented in the 3dfx hardware. So we can now conclude that all screenshot comparisons that don't use the HyperSnap program are invalid ,since they do not give an accurate representation of the true image quality of the 3dfx Voodoo3 board.
I heard that there are different filters ?
Yes, Gary Tarolli has confirmed on the Usenet (and by email) that the Voodoo3 supports 2 types of post filters. One filter is the old Voodoo2 filter, this filter delivers hi-speed average quality images. One typical artifact from this filter is the horizontal lines seen in many on-line screenshots. Since many people complained about this artifact after the release of Voodoo2, the engineers from 3dfx decide to implement a new, better filter. This new filter is slightly slower (should be hardly noticeable) but delivers much better image quality.
So how do I select the better filter ?
According to Gary Tarolli, you should use the check box in the control panel that says "higher quality video" or something like that. According to Usenet feedback, it might be that this check box is corrupted in some driver versions out there. People have reported no difference at all while others have reported a big difference. 3dfx is looking into this, it might even be that this box works reversed (on is off and off is on). Since I don't have a card yet, I can't provide you with more details. Normally, the new filter should reduce the horizontal artifacts of the old filter considerably.
On the next page I have some screenshot details that should
show you that Voodoo3's 16/22'bit quality is very good. Note that
all image comparison conclusions are personal, what one person likes might
not be liked by another. So do make up your own mind based on the
screenshots.
