Scale_Bar_Tool.ijm

// Kevin Terretaz 
//adapted from Aleš Kladnik there https://forum.image.sc/t/automatic-scale-bar-in-fiji-imagej/60774

var REMOVE_SCALEBAR_TEXT = false;

macro "Live Scale Bar Tool - C000 T0508B  T5508a  Ta508r"{
	getPixelSize(unit,w,h);
	if (unit == "pixels") exit("Image not spatially calibrated");
	bar_Length = 1;	// initial scale bar length in measurement units
	bar_Relative_Size = 0;
	bar_Height = 0;
	if (REMOVE_SCALEBAR_TEXT == true) text_Parameter = "hide";
	else text_Parameter = "bold";
	font_Size = minOf(Image.width, Image.height) / 15; // estimation of "good" font size
	getCursorLoc(x2, y2, z2, flags2);
	getCursorLoc(last_x, last_y, z, flags);
	while (flags >= 16) { //left click			
		getCursorLoc(x, y, z, flags);
		getDisplayedArea(area_x, area_y, width, height);
		//if mouse moved
		if (x != last_x || y != last_y) {
			// approximate size of the scale bar relative to image width
			bar_Relative_Size = round(((width-(x - area_x))/width) * 10);

			// recursively calculate a 1-2-5-10-20... series
			// 1-2-5 series is calculated by repeated multiplication with 2.3, rounded to one significant digit
			for (i = 0; i < bar_Relative_Size; i++) {
				magical_Formula = Math.pow( 10, (floor( Math.log10( abs(bar_Length * 2.3)))));
				bar_Length = round( (bar_Length*2.3) / magical_Formula) * magical_Formula;
			}

			bar_Height = round(((height - (y - area_y)) / height) * Image.height/20);

			//if size or height values changed from last loop, update scale bar
			if (bar_Relative_Size != last_Size || bar_Height != last_Height) 
				run("Scale Bar...", "width=&bar_Length height=&bar_Height font=&font_Size color=White background=None location=[Lower Right] "+ text_Parameter +" overlay");
			showStatus("height = "+bar_Height+ "px   length = "+ bar_Length + unit);
			bar_Length = 1;
		}
		//save changes
		last_Size = bar_Relative_Size;
		last_Height = bar_Height;
		getCursorLoc(last_x, last_y, z, flags);
		wait(10);
	}
}

macro "Live Scale Bar Tool Options"{
	REMOVE_SCALEBAR_TEXT = getBoolean("Remove text?", "Yes", "No");
}