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coordinate systems (was: Re: Linear image)

Thanks, this helps a lot. So, just to clarify one thing... If I have an image and want to sample 1 pixel to the right of the current location, I would add 1.0 to x in working coordinates, and run it through samplerTransform to get sampler coordinates. It looks like I've been doing this wrong all along, and it's just happened to work since I never did anything too complex with the image inputs and outputs.
--Sam

Pierre-Olivier Latour wrote:
QC coordinate system is used to position things in the 3D scene:
http://developer.apple.com/documentation/GraphicsImaging/Conceptual/ QuartzComposer/qc_concepts/chapter_2_section_9.html#//apple_ref/doc/ uid/TP40001357-CH202-BCICEEFE

CI coordinate system is only used with CI filters / kernels as they work with images, not the 3D space: it uses pixels with the (0,0) is at the bottom-left corner

Using Image Dimensions, Rendering Destination Dimensions and Math, you can convert from one to the other if necessary: check this composition for example:
/Developer/Examples/Quartz Composer/Interface Builder Compositions/ Mouse Tracking.qtz 

and

John Harper wrote:


Core Image has a couple of conceptual coordinate spaces:

Working Coordinates: this is the coordinate system that images exist in, you can think of it as an infinitely large board on which you pin images. Use the -imageByApplyingTransform: method or the CIAffineTransform filter to move images around in this space.

Sampler Coordinates: only seen inside the kernels, each "sampler foo" variable (i.e. each texture when rendering with OpenGL) represents some finite subregion of the infinite working space. Point 0,0 is the bottom left corner of the physical texture, one unit is one texel.

So this means that whenever you use the sample () function in a kernel, the point you specify must be in the coordinate space of that sampler object, not in the working coordinate space (the actual pixels in the texture may be flipped vertically, or have a non-zero translation when rendering tiled, or maybe a weird scale factor if the engine moved an affine transform around)

These are the functions that deal with this:

destCoord () -> the current output point, in the working coordinate space.

samplerTransform (x, p) -> working space point p, mapped into the coordinate space of sampler x

samplerCoord (x) -> the current output point, mapped into the coordinate space of sampler x, equivalent to samplerTransform (x, destCoord ())

If you want to forget all these details, create a new sampling function:

vec4 sampleWorking (sampler x, vec2 p) {
  return sample (x, samplerTransform (x, p));
}

then just use working space coordinates everywhere.

    John




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References: 
 >Re: Linear image (From: Ralph Brunner <email@hidden>)
 >Re: Linear image (From: Sam Kass <email@hidden>)
 >Re: Linear image (From: Pierre-Olivier Latour <email@hidden>)
 >Re: Linear image (From: John Harper <email@hidden>)



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