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This is a tool for indexing large lists of geographic points or lines and dynamically generating map tiles from the index for display.


  • Modern C compiler like gcc or clang
  • make
  • libpng
  • Ideally a 64 bit machine with >8 GB free memory


First install make and libpng then type:


After the build finishes you will have 4 new command line programs available in the local directory:

encode render enumerate merge


The basic idea is that if you have a file of points like this:


or segments like this:

40.694033,-73.987300 40.693883,-73.987083
40.693883,-73.987083 40.693633,-73.987000
40.693633,-73.987000 40.718117,-73.988217
40.718117,-73.988217 40.717967,-73.988250
40.717967,-73.988250 40.717883,-73.988433
40.717883,-73.988433 40.717767,-73.988550

you can index them by doing

cat file | ./encode -o directoryname -z 16

to encode them into a sorted quadtree in web Mercator in a new directory named directoryname, with enough bits to address individual pixels at zoom level 16.

You can then do

./render -d directoryname 10 301 385 

to dump back out the points that are part of that tile, or

./render directoryname 10 301 385 > foo.png

to make a PNG-format map tile of the data. (You need more data if you want your tile to have more than just one pixel on it though.)

Alternately, if you want an image for a particular area of the earth instead of just one tile, you can do

./render -A -- directoryname zoom minlat minlon maxlat maxlon > foo.png

The "--" is because otherwise getopt will complain about negative numbers in the latitudes or longitudes. For example you could use

./render -A -- 12 37.192596 -122.811526 38.070528 -121.702961 > sf.png

to generate an image of the San Francisco Bay Area at zoom level 12 from the encoded data in

The point indexing is inspired by Brandon Martin-Anderson's Census Dotmap. The vector indexing is along similar lines but uses a hierarchy of files for vectors that fit in different zoom levels, and I don't know if anybody else does it that way.

Rendering assumes it can mmap an entire copy of the file into the process address space, which isn't going to work for large files on 32-bit machines. Performance, especially at low zoom levels, will be much better if the file actually fits in memory instead of having to be swapped in.

Merging files

encode will only write to a brand new file. If you want to add data to an existing file, the way to do it is to create a new file with encode and then use merge to combine the old and the new.

$ cat newdata | encode -o
$ merge -o

merge also has an option, -u, to eliminate duplicates between the source files while merging them.

Generating a tileset

The enumerate and render programs work together to generate a tileset for whatever area there is data for. If you do, for example,

$ enumerate -z14 dirname | xargs -L1 -P8 ./render -o tiles/dirname

enumerate will output a list of all the zoom/x/y combinations that appear in dirname through zoom 14, and xargs will invoke render on each of these to generate the tiles into tiles/dirname.

You can enumerate a single zoom by specifying both -z and -Z for maximum and minimum. So if you want just z12, enumerate -z12 -Z12.

The -P8 makes xargs invoke 8 instances of render at a time. If you have a different number of CPU cores, a different number may work out better.

If you want to filter the output of render, for example through pngquant to reduce the number of colors, you can do it by having xargs invoke a subshell.

$ enumerate -z8 dirname | xargs -L1 -P8 sh -c 'mkdir -p tiles/dirname/$2/$3; render $1 $2 $3 $4 | pngquant 32 > tiles/dirname/$2/$3/$4.png' dummy

The dummy argument is important because sh -c eats the first argument after the command.

Adding color to data

The syntax for color is kind of silly, but it works, so I had better document it.

Colors are denoted by distance around the color wheel. The brightness and saturation are part of the density rendering; the color only controls the hue.

If you want to have 256 possible hues, that takes 8 bits to encode, so you need to say

encode -m8

to give space in each record for 8 bits of metadata. Each input record, in addition to the location, also then needs to specify what color it should be, and the format for that looks like

40.711017,-74.011017 :0
40.710933,-74.011250 :85
40.710867,-74.011400 :170

to make the first one red, the second one green, and the third one blue. And then when rendering, you do

render -C256

to say that it should use the metadata as 256ths of the color wheel.

Options to render

Input file, zoom level, and bounds

The basic form is

render dir zoom x y

to render the specified tile into a PNG file on the standard output.

-A ... dir zoom minlat minlon maxlat maxlon
Instead of rendering a single tile (zoom/x/y), the invocation format changes to render the specified bounding box as a single image.
-f dir
Also read input from dir in addition to the file in the main arguments. You can use this several times to specify several input files.

Output file format

Output plain text (same format as encode uses) giving the coordinates and metadata for each point or line within the tile.
Output GeoJSON giving the coordinates and metadata for each point or line within the tile.
-T pixels
Image tiles are pixels pixels on a side. The default is 256. 512 is useful for high-res "retina" displays.
Leaflet-style retina, where a request for a tile at zoom level N is actually a request for a quarter of a tile at zoom level N-1. In this case, the quarter-tiles remain 256x256.
-o dir
Instead of outputting the PNG image to the standard output, write it in a file in the directory dir in the zoom/x/y hierarchy. It will also write a basic dir/metadata.json that will be used if you package the tiles with mbutil.


-t opacity
Changes the background opacity. The default is 255, fully opaque.
The default background color becomes white, not black.
-b hex
Specifies hex to be the background color. The default is black (or white, if -w is set.)
Makes the output image a mask: The data areas are transparent and the background is opaque. The default is the opposite.


-c hex
Specifies hex to be the fully saturated color at the middle of the output range. The default is gray.
-S hex
Specifies hex to be the oversaturated color at the end of the output range. The default is white.
Use only the color range leading up to full saturation. The default treats saturated color as the middle of the range and allows the output to be oversaturated all the way to white (or the -S color).

Brightness and thickness

-B base:brightness:ramp
Sets the basic display parameters:
  • Base is the zoom level where each point is a single pixel. The default is 13.
  • Brightness is the value contributed by each dot at that zoom level. The default is 0.05917. With the default (square root) gamma, this means it takes 4 dots on the same pixel to reach full color saturation and 16 to reach full oversaturation. (It should have been 0.0625 so that it would hit it exactly.)
  • Ramp is the an additional brightness boost given to each dot as zoom levels get higher, or taken away as zoom levels get lower, slightly increasing the effect of halving the number of dots with each zoom level. The default is 1.23.
-e exponent
Allows specifying a different rate at which dots are dropped at lower zoom levels. The default is 2, and anything much higher than that will look terrible at low zoom levels, and anything much lower will be very slow at low zoom levels. 1.5 seems to work pretty well for giving a quality boost to the low zoom levels. The ramp from -B is automatically adjusted to compensate for the change.
-G gamma
Sets the gamma curve, which causes each additional dot plotted on the same pixel to have diminishing returns on the total brightness. The default is 0.5, for square root.
-L thickness
Sets the base thickness of lines. The default is 1, for a single pixel thickness.
-l ramp
Sets the thickness ramp for lines. The line gets thicker by a factor of ramp for each zoom level beyond the base level from -B. The default is 1, for constant thickness. Thicker lines are drawn dimmer so that the overall brightness remains the same.
-p area
Specifies a multiplier for dot sizes. Point brightness is automatically reduced by the same factor so the total brightness remains constant, just diffused. The default is 1. (Example -p5 for area 5)
-p garea
Specifies a Gaussian brush instead of a flat disk, as well as a multiplier for dot sizes. (Example: -pg5 for Gaussian with area 5)


-C hues
Interpret the metadata as one of hues hues around the color wheel. Numbering starts at 0 for red and continues through orange, yellow, green, blue, violet, and back to red.
-C meta1:hue1:meta2:hue2
Specify a range of hues that correspond to a domain of meta values. The hues are numbered in degrees: 0 for red, 30 for orange, 60 for yellow, 120 for green, 180 for cyan, 240 for blue, 300 for violet. You can specify hues below 0 or above 360 to wrap around across red.
-x cradiusf / -x cradiusm
Interpret the metadata as a number of points to be plotted in the specified radius (in feet or meters) around the point in the data.
-x b
Make the brightness of each feature proportional to the metadata value.
-x r
Make the radius of each point proportional to the metadata value.
-x smax
Cap the saturation of meta colors at max instead of 0.7. They will go all the way to white if you use 1.
-x u
Use an approximation of CIELCH uniform color space so that all hues with the same density will have approximately equal lightness and saturation. Blues will be brighter and greens will be dimmer.


Reduce the brightness of lines whose endpoints are far apart, to compensate for GPS samples that jump around, or bogus connections to to 0,0.
-O base:dist:ramp
Tune the parameters for reasonable distances between points:
  • Base is the zoom level at which only fully acceptable samples are given full brightness. The default is 16.
  • Dist is the allowable distance between samples at the base zoom level. The unit is z32 tiles, or about 1cm, and I need to make that something more human-oriented. The default is 1600.
  • Ramp is the factor of additional distance that is allowed at each lower zoom level. The default is 1.5.
-M latitude
Mercator compensation. Makes the dots bigger at latitudes higher than the one specified and smaller at latitudes closer to the equator.

Vector styling

Instead of the normal output, produce a CartoCSS file for TileMill 2 to approximate the brightness, dot ramp, gamma, and colors you specified. Use render-vector to make the vector tiles themselves.


Turn off anti-aliasing


Indexes points and lines and generates map tiles to display them




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