Add a transfer function shader widget

[seankmartin]

Adds a new shader widget directive transferFunction. Transfer functions can be used to map specific data values to color and opacity values via a set of control points. In between control points, the values are linearly interpolated.

A preview of the underlying lookup texture is shown in the UI panel for a transfer function.

Points are added to the panel via a click, their color is changed by setting a new color in the picker, and then right-clicking the point. Points are removed via a modifier and double-click on the point.

[chrisj]

@seankmartin could you share a ng link that demonstrates the feature? I tried to get a basic grayscale image out of it but I think I'm missing something

https://neuroglancer-demo.appspot.com/#!%7B%22dimensions%22:%7B%22x%22:%5B1.6e-8%2C%22m%22%5D%2C%22y%22:%5B1.6e-8%2C%22m%22%5D%2C%22z%22:%5B4e-8%2C%22m%22%5D%7D%2C%22position%22:%5B32102.173828125%2C15304.3671875%2C3552.684326171875%5D%2C%22crossSectionScale%22:0.6126263941844161%2C%22projectionScale%22:65536%2C%22layers%22:%5B%7B%22type%22:%22image%22%2C%22source%22:%22precomputed://https://bossdb-open-data.s3.amazonaws.com/flywire/fafbv14%22%2C%22tab%22:%22rendering%22%2C%22shader%22:%22#uicontrol%20transferFunction%20colormap%28window=%5B0%2C%20100%5D%2C%20channel=%5B%5D%2C%20defaultColor=%5C%22#000000%5C%22%29%5Cn%5Cnvoid%20main%28%29%20%7B%5Cn%20%20emitRGBA%28colormap%28%29%29%3B%5Cn%7D%5Cn%22%2C%22shaderControls%22:%7B%22colormap%22:%7B%22controlPoints%22:%5B%5B68%2C%22#000000%22%2C0.5137254901960784%5D%2C%5B94%2C%22#ffffff%22%2C0.8313725490196079%5D%5D%7D%7D%2C%22name%22:%22fafbv14%22%7D%2C%7B%22type%22:%22image%22%2C%22source%22:%22precomputed://https://bossdb-open-data.s3.amazonaws.com/flywire/fafbv14%22%2C%22tab%22:%22rendering%22%2C%22shaderControls%22:%7B%22normalized%22:%7B%22range%22:%5B0%2C200%5D%7D%7D%2C%22name%22:%22fafbv141%22%2C%22visible%22:false%7D%5D%2C%22selectedLayer%22:%7B%22size%22:906%2C%22visible%22:true%2C%22layer%22:%22fafbv14%22%7D%2C%22layout%22:%22xy%22%7D

[seankmartin]

Hey @chrisj, for sure, no worries!

From the link you sent, I've created a transfer function that mimics an invlerp so you can compare here

More generally, you can add points by clicking, change their color by right clicking after selecting a color in the color picker, and remove points by shift double clicking for something like this

At the moment, the transfer function does not show any indication of the underlying data distribution, which can make creating them a little tricky. Let me know if you have questions, happy to help