Re: colors and contours in luametafun

[Jacob Kauffmann via ntg-context <ntg-context@ntg.nl>]

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Dear list,

> Am 01.09.2021 um 09:54 schrieb Jacob Kauffmann via ntg-context <ntg-context@ntg.nl>:
> - color by value in lmt_surface
> We want to show a surface plot of a function where the color indicates the function value (instead, or in addition to a light source).
> The easiest would be to have variable cf for the brightness factor (as it is), and in addition variables such as zt, minz, maxz, akin to the color formatter for lmt_contour.
> This becomes handy when colors are to be aligned with function values (e.g. negative = blue, zero = white, positive = red, see MWE attached).

I managed to achieve this by tweaking f_color and mp.lmt_surface_do in mlib-cnt.lmt in
	ConTeXt  ver: 2022.05.11 11:36 LMTX  fmt: 2022.6.20
a bit.
Please find the diff and a MWE attached.

Cheers,
Jacob Kauffmann

--- context/tex/texmf-context/tex/context/base/mkxl/mlib-cnt.lmt   2022-06-20 18:11:51.000000000 +0200
+++ mlib-cnt.lmt        2022-06-20 18:26:22.000000000 +0200
@@ -1839,7 +1839,29 @@

 local f_color = formatters [ [[
     local math = math
-    return function(f)
+    local min  = math.min
+    local max  = math.max
+    local abs  = math.abs
+    local minz = %s
+    local maxz = %s
+
+    local color_value = 0
+    local color_step  = mp.lmt_color_functions.step
+    local color_shade = mp.lmt_color_functions.shade
+
+    local function step(...)
+        return color_step(color_value,n,...)
+    end
+    local function shade(...)
+        return color_shade(color_value,n,...)
+    end
+    -- local function lin(l)
+    --     return l/n
+    -- end
+    %s
+    return function(z, f)
+        function_value = z
+        brightness_factor = f
         return %s
     end
 ]] ]
@@ -1898,7 +1920,7 @@
     local dy   = (ymax - ymin) / ny
     local xt   = xmin
     --
-    local minf, maxf
+    local minf, maxf, minz, maxz
     --
     -- similar as contours but no data loop here
     --
@@ -1908,12 +1930,6 @@
         return false -- fatal error
     end
     --
-    local ccode = load(f_color(colorcode))
-    local color = type(ccode) == "function" and ccode()
-    if type(color) ~= "function" then
-        return false -- fatal error
-    end
-    --
     for i=0,nx do
         local yt = ymin
         for j=0,ny do
@@ -1946,7 +1962,16 @@
                 maxf = fac
             end
             --
-            data[i][j] = { x, y, fac }
+            if not minz then
+               minz = z
+               maxz = z
+            elseif z < minz then
+               minz = z
+            elseif z > maxz then
+               maxz = z
+            end
+            --
+            data[i][j] = { x, y, fac, z }
             --
             yt = yt + dy
         end
@@ -1957,6 +1982,16 @@
     local range   = maxf - minf
     local cl      = linecolor or 1
     local enforce = attributes.colors.model == "cmyk"
+    --
+    -- local ccode = load(f_color(colorcode))
+    report("minz, maxz is (%s,%s)",minz,maxz)
+    report("colorcode is %s", colorcode)
+    local ccode = load(f_color(minz, maxz, preamble, colorcode))
+    local color = type(ccode) == "function" and ccode()
+    if type(color) ~= "function" then
+        return false -- fatal error
+    end
+    --
     for i=0,nx-1 do
         for j=0,ny-1 do
             -- points
@@ -1985,12 +2020,13 @@
             local z32 = z3[2]
             local z41 = z4[1]
             local z42 = z4[2]
+            local zout = z1[4]
          -- if lines then
          --     -- fill first and draw then, previous shapes can be covered
          -- else
          --     -- fill and draw in one go to prevent artifacts
          -- end
-            local cr, cg, cb = color(cf)
+            local cr, cg, cb = color(zout, cf)
             if not cr then cr = 0 end
             if not cg then cg = 0 end
             if not cb then cb = 0 end


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\starttext

\setupMPpage[
	offset=10pt,
	instance=doublefun]
	
\startluacode
-- function used in the examples
function fun(x,y)
	local exp = math.exp
	return 1.5*exp(-1.5*((x + 1.1)^2 + (y - 1.1)^2)) 
		 + 0.5*exp(-1.0*((x + 1)^2 + (y + 1)^2))
		 - 0.5*exp(-0.9*((x - 1)^2 + (y - 0)^2))
end

-- example of a simple blue-gray-red colormap
function colormap(f)
	local abs, min = math.abs, math.min
	local r, g, b = min(1, 2*f), 1-2*abs(f-.5), min(1,2-2*f)
	return .85*r, .85*g, .85*b
end
\stopluacode

\startMPpage
draw lmt_surface [
	code = "fun(x,y)",
	color = "colormap( 0.5 * z / max(abs(minz),abs(maxz)) + 0.5 )",
	% color = "colormap(f)",
	xmin = -3,
	xmax = 3,
	ymin = -3,
	ymax = 3,
	xstep = .3,
	ystep = .3,
	xvector = { -0.3, -0.5 },
	linecolor = 0
] xsized 10cm;
\stopMPpage

\stoptext

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