You are probably already familiar with the following example or something similar.
However, given the rate that historical data seems to deteriorate in these parts, I'll dump it anyway:

(note: I am not the author.
Modifications might have been made from original.)
Code:

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var terrain_chunk = 0;		// won't work in 6.40.5 unless you do this

// (Loopix-ShadowmapAmbient0)
// Terrain Brightness
// Credit appreciated...
// src author: Josh Miller (xXxGuitar511)
// tD arc date: 2006-06-09
// tD mod possible
// ver: 6.60, 6.40.5?

define	TraceDist,	100;	// Distance to scan down...
define	FloorOffs,	10;		// Height above entities feet to trace from
define	IsTerrain,	flag5;	// Set this flag for all terrain entities
//define	SkinNumber,	3;		// Skin to scan for brightness (red channel)
define SkinNumber, 2;
define	BrightScale,100;		// Percentage (0 - 100) of brightness
define	BrightOffs,	0; 	// Offset of brightness (from 0)

var ScanFrom[3];
var ScanTo[3];
var ScanDist;
var TargetX;
var TargetY;
var TargetPixel;
var PixelColor[3];
var BMapFormat;
bmap* TerrainSkin;

var scanFloorEnable = 0;

action terrain0 {
	my.skin = SkinNumber;
	my.IsTerrain = on;
}

function ScanFloor();	// *** Prototype function...



function ScanFloor()
{
	vec_set(ScanFrom, my.x);
	ScanFrom[2] += my.min_z + FloorOffs;
	vec_set(ScanTo, ScanFrom);
	ScanTo[2] -= TraceDist;
	ScanDist = c_trace(ScanFrom, ScanTo, ignore_me | ignore_passable | ignore_passents);
	//trace_mode = ignore_me + ignore_passable + ignore_passents;
	//ScanDist = trace(ScanFrom, ScanTo);
	if (ScanDist == 0)
	{ScanDist = TraceDist;}
	if (you != null) {
		if (you.IsTerrain == on) {
			// Is a terrain
			TerrainSkin = bmap_for_entity(you, SkinNumber);
			BMapFormat = bmap_lock(TerrainSkin, 0);
			if (TerrainSkin == 0) { goto(NoTerrain); }
			vec_for_vertex(ScanFrom, you, 1);
			vec_for_vertex(ScanTo, you, ent_vertices(you));
			TargetX = (my.x - ScanFrom.x) / (ScanTo.x - ScanFrom.x) * bmap_width(TerrainSkin);
			TargetY = (my.y - ScanFrom.y) / (ScanTo.y - ScanFrom.y) * bmap_height(TerrainSkin);
			TargetX = clamp(int(TargetX), 0, bmap_width(TerrainSkin));
			TargetY = clamp(int(TargetY), 0, bmap_height(TerrainSkin));
			TargetPixel = pixel_for_bmap(TerrainSkin, TargetX, TargetY);
			if (TargetPixel == -1)
			{goto(NoTerrain);}
			pixel_to_vec(PixelColor, null, BMapFormat, TargetPixel);
			bmap_unlock(TerrainSkin);
			my.ambient = clamp((PixelColor.blue / 255 * BrightScale) + BrightOffs, 0, 100);
			//my.ambient = (PixelColor.red + PixelColor.green + PixelColor.blue) / 765 * 100;
		}
	}
	//
NoTerrain:
	//
	return(ScanDist);
}

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