For the issue at q = 0, setting your samplers addressing modes (AddressU, AddressV) to clamp does the trick. Values 'near' 0 (q = 0.5) are trickier, since you can't really tell when they happen with some epsilon. These continuity things go wrong in the sampler, so when the sample reaches your shader it'll already have been interpolated to the wrong value. You could check for discontinuities by sampling neighboring pixels, but then you might as well jump straight to implementing linear filtering.

Alternatively you could render the signed 16 bit image to an unsigned rendertarget of the exact same size with point filtering. Next you render this result to your target surface with linear filtering, giving you hardware interpolation over the correct values. It's a detour, but it's correct and should still be quite fast. Whether it's feasible for your usecase with a larger number of quads however, I can't say.

Quote:Original post by dario_ramos
I thought you meant I had to set those modes using SetSamplerState when initializing the device. I did that and nothing changed.

I did mean that, but it seems that particular issue I had around q=0 is no problem in your scenario and we're just stuck with the tricky one around q=0.5.

Quote:However, I looked around some more about this continuity issues and found that, aside from tex2D, there are tex2Dlod and tex2dgrad functions. When using those, it seems you can change sampler states inside the pixel shader. Is that true? I read tex2Dlod and tex2Dgrad's documentation but didn't understand how to apply them to my problem... Is that possible?

As far as I know, tex2Dlod and tex2dgrad are a lot less magical than that. If I understand them correctly, these are meant to be used as replacements for tex2D when you need to call it from within some kind of flow control or branching in your pixel shader.

Normally the sampler can judge which mip level you'd want by the (2nd?) derivate of the texture coordinates you pass to tex2D. With tex2D instructions within flow control however (for example if statements) this derivate can't be depended on since it may differ wildly from pixel to pixel, so you use these to help the sampler out in selecting a mip level.

We shouldn't need tex2D instructions within flow control though and you probably shouldn't be using any mip levels anyway if you want to use your image at maximum resolution, so these are of no use here I think. I could be way off here though, so please correct me if I'm wrong.

Quote:But I'm still getting those nasty artifacts...

Yep, same here with my little test project. I tried a number of things but the problem remains that the sampler simply returns wrong interpolations when using anything but point filtering. Crappy continuity checking introduced more artifacts than it fixed, but I tried implementing some simple linear filtering which does work a bit:

    point         naive filter

It's still a far cry from the default hardware linear filtering though. So either I messed something up, or the hardware filtering is a lot more complex and/or adaptive than my simple naive filter.

With this said and done, I still think your best bet is the 2 'pass' approach described above, where you convert from signed to unsigned in the 1st pass and let the shader do hardware linear filtering on the correct unsigned values in the 2nd. Multiframe image playback need not be a showstopper usecase, since even with this 2 pass approach any non-ancient graphics hardware should be able to render this in the order of hundreds of frames per second.

This topic is haunting me [smile] I realized over coffee this morning that we've been sitting on the simple answer all along:

Quote:Original post by remigius
... the problem remains that the sampler simply returns wrong interpolations when using anything but point filtering ...

By setting up 2 samplers, one with point filtering and one with linear filtering, you can just take a point sample to determine if the remapped value is in a problem area (q=0.5). If so you can use the point sample there and if not, just use the linear sample.



Sorry about the detour, I hope this is still of some use.


[Edited by - remigius on February 11, 2010 1:40:21 AM]

Sneaky :D

I don't have any experience using the default samplers from my pixel shader, I just set them up in my effects file. Here are the samplers I defined in my test project. I'm not sure it's good form to use the same Texture in 2 samplers, but it seems to work. This goes into the top part of your effects (shader) file:



It's probably the best idea to set the texture directly onto the effect, instead of mucking about with registers to set it through the device. I'm not sure how this works in unmanaged D3D, but ID3DXBaseEffect::SetTexture sounds like a good bet [smile]