LightMap

The LightMap applet takes a color texture and a grayscale bump map to calculate a real-time lit surface using 2D bump mapping. A radial light source follows the mouse cursor across the applet area; when the mouse leaves, the light orbits automatically along a smooth elliptical path. The bump map is converted into per-pixel x/y gradient shifts via finite differences, and for each pixel the light intensity is looked up from a pre-computed radial falloff table. The final color is the texture color multiplied by that intensity — all done per frame using integer shifts for speed, producing true-color output.

Bump mapping with dynamic light sources was a signature effect of the Atari ST, Amiga and early PC demoscene in the mid-to-late 1990s. On the 68000 CPU the per-pixel gradient lookup and light multiply were carefully arranged to avoid expensive MUL instructions — bump shifts were stored in pre-computed tables and the RGB scaling used shifts and adds instead of full multiplications. On the x86 side, 386/486 demos exploited 32-bit registers to pack two color channels and process them in a single operation, while Pentium coders paired instructions to keep both U and V pipelines busy. This Java applet follows the same philosophy: the >> 8 shift replaces a division by 256 in the per-pixel lighting, and the radial falloff is read from a pre-computed lookup table rather than calculated on the fly.

Features

Real-time 2D bump mapping with radial point light
Interactive: light follows mouse cursor, auto-orbits when mouse leaves
Pre-computed radial falloff table for the light source
Bump gradients calculated via finite differences on the height map
Per-pixel lighting: texture RGB multiplied by light intensity using integer shifts
True-color (32-bit ARGB) output
Automatic image rescaling to applet dimensions
Optional clickable link with URL shown in status bar on mouseover

Parameters

Name	Type	Description	Default	Required
`pic`	URL	Color texture image (GIF or JPEG)	–	yes
`bump`	URL	Grayscale bump/height map image	–	yes
`link`	URL	Link target when applet is clicked	–	no
`targetWindow`	String	Target window for link (`_blank`, `_parent`, etc.)	–	no
`movement`	flag	If present, light auto-orbits (no value needed)	–	no
`lightSize`	int	Diameter of the light source (4–256)	128	no
`ambient`	int	Ambient light level (0–128, 0 = no ambient)	8	no

How It Works

On startup the applet pre-computes a radial light falloff table:

for each (x, y) in lightSize × lightSize:
  r = distance² from center
  if r < radius²:
    field = (1024 - 1024 * r / radius²)² >> 12
  else:
    field = 0

This gives a smooth inverse-square-like falloff clamped to 0–255.

The bump map is converted to gradient vectors using finite differences:

xShift[pos] = (height[pos+1] - height[pos-1]) / 2
yShift[pos] = (height[pos+w] - height[pos-w]) / 2

Each frame, for every pixel the light position is offset by the bump gradient, then the pre-computed falloff value is used to shade the texture color:

intensity = lightBall[lightPos - bumpShift]
r = (intensity * textureR[pos]) >> 8
g = (intensity * textureG[pos]) >> 8
b = (intensity * textureB[pos]) >> 8

The >> 8 shift replaces a division by 256, keeping the per-pixel math entirely in integer arithmetic.

Code Example

<applet archive="LightMap.jar"
  code="LightMap.class" width="320" height="150">
  <param name="pic" value="logo.jpg">
  <param name="bump" value="bump.jpg">
  <param name="movement">
  <param name="link" value="https://www.chriscohnen.de">
</applet>