subsample.py

import rasterio
import numpy as np

src = rasterio.open('/Users/sean/code/decloud/envs/1_16/output/0_q.tif')

# Fraction of elements with value > 100 at full resolution.
q = src.read_band(1)
ones = np.ones(q.shape)
print "Full, %d pixels" % (q.shape[0]*q.shape[1])
print sum(ones[q>100])/(q.shape[0]*q.shape[1])

# Fraction of elements with value > 100, sampling only 1 in 16.
shape = (q.shape[0]/4, q.shape[1]/4)
q = np.zeros(shape, dtype=np.uint8)
q = src.read_band(1, out=q)
ones = np.ones(q.shape)
print "Subsampled, %d pixels" % (q.shape[0]*q.shape[1])
print sum(ones[q>100])/(q.shape[0]*q.shape[1])

# Output
Full, 6806881 pixels
1683126000.0  m^2
Subsampled, 425104 pixels
1681646400.0  m^2