Panel pedestal does not work?

[dagewa]

I've just come across the panel pedestal attribute. This seems to be a fairly recent innovation (this year at least: f96a16d) but as far as I can tell it does not work.

The "pedestal" should be subtracted as a dark image here:

dxtbx/imageset.h

Lines 680 to 685 in d95a922

	// Apply dark
	if (p.size() > 0) {
	for (std::size_t j = 0; j < r.size(); ++j) {
	c[j] = c[j] - p[j];
	}
	}

however, even if panel.pedestal is set to something non-zero, this block is never called because p.size() == 0.

[dagewa]

Perhaps this is at fault

dxtbx/imageset.h

Lines 770 to 772 in d95a922

	} else if (std::abs(pedestal[i]) > 1e-7) {
	need_pedestal_map = false;
	break;

Should be need_pedestal_map = true;?

[dagewa]

I note also that the format classes that use a pedestal (typically FormatSMV etc.) don't use this mechanism and instead subtract a pedestal from get_raw_data, which is wrong

[dagewa]

Another point: the code here only allows for strictly positive pedestal maps. But for some detectors in electron diffraction there may be negative bias in the images. I would therefore like to add a uniform positive value to the images, with the value chosen by the user. The panel.pedestal attribute would appear to be the way to do that, by supplying a negative pedestal value.

The alternative, modifying the behaviour of get_raw_data under the control of an environment variable is fairly horrible.