From: Martin DeMello <martindemello@...>
Date: 2007-02-08T04:25:22+09:00
Subject: Re: Algorithm Help - Grid Space Calculations

Here you go:

include Math

BASE_GRID_SIZE = 40

def lg2(a)
	log(a) / log(2)
end

def grid_size( world_width, pixel_width )
	ppu = pixel_width / world_width
	factor = 2 ** lg2(ppu).ceil
	low = BASE_GRID_SIZE / factor
	high = low * 2
	$grid_spacing = ($grid_spacing - high).abs < ($grid_spacing -
low).abs ? high : low

	px_per_grid_line = $grid_spacing * ppu

	puts "A %dcm wide camera mapped to %dpx " % [world_width,pixel_width] +
				"will have a grid every %dcm " % [$grid_spacing] +
				"drawn every %.2f pixels, " % [px_per_grid_line] +
				"resulting in %.2f lines being drawn." % [pixel_width/ px_per_grid_line]
	$grid_spacing
end

require 'test/unit/assertions'
include Test::Unit::Assertions

begin
	# Use a global to handle overlapping legal ranges
	$grid_spacing = BASE_GRID_SIZE

	# Changing window size
	assert_equal( BASE_GRID_SIZE, grid_size( 400.0, 400 ) )
	assert_equal( BASE_GRID_SIZE*2, grid_size( 400.0, 190 ) )
	assert_equal( BASE_GRID_SIZE/2, grid_size( 400.0, 810 ) )
	assert_equal( BASE_GRID_SIZE/2, grid_size( 400.0, 1200 ) )
	assert_equal( BASE_GRID_SIZE/2, grid_size( 400.0, 1600 ) )
	assert_equal( BASE_GRID_SIZE/4, grid_size( 400.0, 1601 ) )

	# Changing camera zoom
	assert_equal( BASE_GRID_SIZE, grid_size( 420.0, 400 ) )
	assert_equal( BASE_GRID_SIZE, grid_size( 790.0, 400 ) )
	assert_equal( BASE_GRID_SIZE*2, grid_size( 810.0, 400 ) )

	# Overlapping range test
	assert_equal( BASE_GRID_SIZE*2, grid_size( 1595.0, 400 ) )
	assert_equal( BASE_GRID_SIZE*4, grid_size( 1610.0, 400 ) )
	assert_equal( BASE_GRID_SIZE*4, grid_size( 1595.0, 400 ) )
	assert_equal( BASE_GRID_SIZE*4, grid_size( 810.0, 400 ) )
	assert_equal( BASE_GRID_SIZE*2, grid_size( 790.0, 400 ) )
	assert_equal( BASE_GRID_SIZE*2, grid_size( 400.0, 400 ) )
	assert_equal( BASE_GRID_SIZE, grid_size( 399.0, 400 ) )
	assert_equal( BASE_GRID_SIZE, grid_size( 790.0, 400 ) )
rescue Test::Unit::AssertionFailedError => e
	puts e
end

martin