What is the "clip" feature for color ramps?

Question

In the singleband pseudocolor band rendering, there is a Boolean checkbox "clip". What does this do? I don't notice a difference in my maps and haven't found documentation for it.

Answer 1

You're right, there's no mention of it in the user or developer documentation. An issue has recently been raised to get this setting documented.

From looking at the source code, the checkbox value maps onto mClip of the QGSColorRampShader class.

According to the comment

// Values outside total range are rendered if mClip is false

It seems this is used to decide on whether or not to render 'out of range' pixel values - those less than the minimum ramp value, or more than the maximum. It's only used for Interpolated, not for Discrete or Exact.

Presumably this only affects things if you've manually set too high a minimum value for your first ramp entry, or too low a maximum value for the last ramp entry, compared to the range of values in your raster.

That seems to be the intent.

However, I don't see any difference either.

I set up those sorts of conditions, and saved two images, one with it checked and one with it not checked - the results were identical

• import as two layers to GIMP,
• made top one difference mode,
• merged down,
• got a uniformly black raster - that means the images are identical

I'm not a C++ developer, so I can only debug-by-eyeball in this case, but it might be a bug - but I'm not in a position to establish this at the code level.