Create Tone Curve Filter in CoreImage

I try to write ToneCurveFilter using CoreImage. I have curves data in RGB image. Size of the RGB image with curves data is 256*1. Every pixel presents curve value. For example, for red = 200, you should get pixel with coordinates (200, 0) and get red channel from this pixel.

Image: http://i.stack.imgur.com/Njhr6.png

I exam colour of every pixel in the curve image, and they are correct. (Identical values, result filter shouldn't change image colour)

Also I write a kernel for the filter:

kernel vec4 coreImageKernel(uniform sampler src, __table sampler toneCurveData)

{

vec4 color = unpremultiply(sample(src, samplerCoord(src)));

//return vec4(color.r, color.g, color.b, 1.0);

vec2 redPointPosition = samplerTransform(toneCurveData, vec2(color.r * 255.0, 0.5));

float red = (sample(toneCurveData, redPointPosition)).r;

vec2 greenPointPosition = samplerTransform(toneCurveData,vec2(color.g * 255.0, 0.5));

float green = (sample(toneCurveData, greenPointPosition)).g;

vec2 bluePointPosition = samplerTransform(toneCurveData,vec2(color.b * 255.0, 0.5));

float blue = (sample(toneCurveData, bluePointPosition)).b;

vec4 resultColor = vec4(red, green, blue, color.a);

return premultiply(resultColor);

}

I expect that this filter, will not change my source image. But a result image is more darker then the original image.

I've tried change color.x * 255.0 to concreate value, for example to 230.0, but color of the result image was (224, 223, 224). And I do not understand why.

To test my kernels I use "Quartz Composer" -- very useful tool.

I've tried add ROI function, but this is not helped for me.

function myROIFunction(samplerIndex, dstRect, __image info) {

__vec4 dstRectResult = dstRect;

if(samplerIndex == 1)

{

dstRectResult = info.extent;

}

return dstRectResult;

}

function __image main(__image src, __image toneCurveData, __color monohromeColor, __number power) {

coreImageKernel.ROIHandler = myROIFunction;

return coreImageKernel.apply(src.definition, toneCurveData, src, toneCurveData, monohromeColor, power);

}

I think problem in coordinate space of toneCurveData sample, but I can't understand what exactly.

Main question: how get a pixel colour from toneCurveData for some pixel (for example, for pixel with coordinates (255.0, 0.0), it should be (255.0, 255.0, 255.0))?

Also I've tryed use a NSData object with toneCurve data as parameter of the kernel, this's not helped me.