[wip] Node.js streams for reading/writing/transforming pixels using node-gdal
Create streams that read pixels from each band of a source image:
var pixels = require('@mapbox/geo-pixel-stream');
var dcrgb = 'node_modules/mapnik-test-data/data/geotiff/DC_rgb.tif';
var readers = pixels.createReadStreams(dcrgb);If an image has 4 bands (e.g. RGBA), then the streams array will contain four stream objects. Each stream contains metadata about the original datasource and the band represented:
console.log(readers[0].metadata);
// {
// driver: 'GTiff',
// width: 1541,
// height: 1913,
// numBands: 4,
// srs: gdal.SpatialReference.fromEPSG(26918),
// geotransform: [ 326356, 4.00129785853342, 0, 4318980, 0, -4.0015682174594875 ],
// id: 1,
// type: 'Byte',
// blockSize: {
// x: 1541,
// y: 1
// }
// }Each PixelReader is a Node.js readable stream. The stream's data event will emit objects indicating the offset (in terms of blocks, not pixels), block size (in pixels) and a TypedArray of pixel data.
readers[0].once('data', function(data) {
console.log(data);
// {
// offset: { x: 0, y: 0 },
// blockSize: { x: 1541, y: 1 },
// buffer: [ Uint8TypedArray ]
// }
});Inheriting from the stream api, PixelReaders can send data to writable streams via pipe:
var writable = new stream.Writable();
readers[1].pipe(writable);Create streams that write pixels from each band of a destination image:
var outputFile = '~/some.tif';
var writers = pixels.createWriteStreams(outputFile);Again, if this will generate one stream for each band of the output file. If, instead of writing to an existing file, you want to write to a new, blank image, you must provide sufficient metadata to generate the new image:
var outfile = '~/just-red-pixels.tif'
var outputMetadata = {
driver: 'GTiff',
width: 1541,
height: 1913,
numBands: 1,
srs: gdal.SpatialReference.fromEPSG(26918),
geotransform: [ 326356, 4.00129785853342, 0, 4318980, 0, -4.0015682174594875 ],
id: 1,
type: 'Byte',
blockSize: { x: 1541, y: 1 }
};
var writers = pixels.createWriteStreams(outputFile, outputMetadata);Once you've created readers and writers, you can pipe data from one image to another:
var dcrgb = 'node_modules/mapnik-test-data/data/geotiff/DC_rgb.tif';
var readers = pixels.createReadStreams(dcrgb);
var readRedBand = readers[0];
var writers = pixels.createWriteStreams(outputFile);
var writeRedBand = writers[0];
readRedBand.pipe(writeRedBand).on('finish', function() {
console.log('All done!');
});Create a stream that takes input pixels, performs some sort of manipulation of them, and outputs the adjusted pixels. You create a function that will receive a TypedArray of pixels, performs some adjustment, and provides the adjusted pixels to the provided callback function:
function processPixels(buffer, callback) {
var result = buffer.map(function(pixel) {
return pixel + 10;
});
callback(null, result);
}
var transform = pixels.createTransformStream(processPixels);If processing encounters an error, send the error as the first argument to the provided callback function.
You can use a transform stream to make adjustments to pixel values before writing them to a destination file:
var dcrgb = 'node_modules/mapnik-test-data/data/geotiff/DC_rgb.tif';
var readers = pixels.createReadStreams(dcrgb);
var writers = pixels.createWriteStreams(outputFile);
var transform = pixels.createTransformStream(processPixels);
readers[0].pipe(transform).pipe(writers[0]).on('close', function() {
console.log('All done!');
});
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