which settings in b6 can tweak the “Tonemapping function”?
All the Film settings directly define the tonemapping curve which transforms the linear HDR input color to linear LDR output. The ColorGrading happens after the conversion from linear LDR to perceptual LDR.
Second group of questions
Here is a breakdown of the Film options,
Tint → Change the white point of the input color. This tint is normalized for intensity. So for instance tinting blue will not darken the scene.
Tint Shadow → A second Tint applied to the darks of the scene again changes color without changing intensity.
Tint Shadow Blend → Controls the quickness of the transition between Tint and Tint Shadow.
Tint Shadow Amount → Controls how much Tint Shadow to use, zero turns off the Tint Shadow (can be faster).
Saturation → As you would expect but saturation is done in linear HDR instead of LDR. This results in very pure hues on high amounts of saturation.
Channel Mixer Red/Green/Blue → This controls the ratio of the primary color input used to produce each primary color output before the tone-mapping. This is applied in linear HDR as well. The channel mixer can be used to do a subset of color grading style of effects common in movies.
Contrast → Controls the slope of the tone-mapping curve. This sets the slope of the tone-mapper curve around 18% grey (the perceptual midpoint in a linear colorspace). The tone-mapper curve has an adjustable size linear section to maximize the area of the tonal curve which has no distortion on the color ratio (think high latitude film).
Crush Shadows → Controls the length of the linear section of the tone-mapper curve on the shadow side of 18% grey. A maximum value here maximizes the linear section which crushes the shadows. A minimum value makes for a more gradual transition at the expense of non-linear color distortion (intensity effects the tint and saturation of the color more).
Crush Highlights → Controls the length of the linear section of the tone-mapper curve on the highlight side of 18% grey. A maximum value here maximizes the linear section which crushes the highlights such that the effect is closer to clipping them off. A minimum value here minimizes the linear section which results in a more gradual transition at the expense of non-linear color distortion.
Dynamic Range → This controls the amount of dynamic range before the tone-mapping curve starts to clip. A setting of 1 would be no dynamic range. Eye adaption happens before the input to Film post so dynamic range is the range after adaption.