Get numpy array using readPixels

development python
#1

Hi there,

(I’m a snappy newbie. ;-))
I am trying to get a numpy array from a S3A_SY_2_VGP product.
It works for one line, or a rectangle starting in the top left corner, but it won’t work for a rectangle somewhere in the middle.
This is (part of) my code:

import numpy as np
from snappy import ProductIO
testfile=’…/S3A_SY_2_VGP____20161101T061252_20161101T065709_20171209T023146_2657_010_248______LR1_R_NT_002.SEN3/xfdumanifest.xml’

product=ProductIO.readProduct(testfile)
width=product.getSceneRasterWidth()
height=product.getSceneRasterHeight()

b0=product.getBand(‘B0’)

dataline=np.zeros(width)
line=b0.readPixels(0,0,width,1,dataline) #this seems to works fine

databox=np.zeros(100*100)
box=b0.readPixels(0,0,100,100,databox) #this works too

databox2=np.zeros(100*100)
box2=b0.readPixels(100,100,200,200,databox2) #this won’t work!

I get an error message:

RuntimeError: java.lang.IllegalArgumentException: The length of the given array is less than 40000

What are the correct arguments for readPixels() ? I interpreted it as (startcolumn, startrow, endcolumn, endrow, outputarray), but this seems incorrect.

Thanks in advance for your help.

#2

Hi cartot,
I’ve started to use snappy some months ago and this is a problem I also got stuck into at the beginning.
I solved this and many other issues by looking at the snappy API reference page --> (http://step.esa.int/docs/v5.0/apidoc/engine/)

By the way, the arguments of readPixels are not as you interpreted but are readPixels(x,y,w,h,Array) with:

  • x: x offset of upper left corner
  • y: y offset of upper left corner
  • w: width of the desired data array
  • h: height of the desired data array
  • Array: the Output Array to be filled with data

So in your case, if you want to get pixels data inside a m x n rectangle, you have to set w = m and h = n inside readPixels (no matter which offset you select), thus:

databox2=np.zeros(100*100)
box2=b0.readPixels(100,100,100,100,databox2)

will do the job!

Hope this helps :wink:
Davide

2 Likes
#3

Hi Davide,
It works! :clap:
Thanks a lot. This snappy API reference page will be very helpful…
Carolien

#4

Hi, for further examples and applications of snappy you can refer to this exteranl resource in the STEP website : https://github.com/techforspace/sentinel

The repository is not rich at the moment, but it is still active and more elements will be added in the next months
Hope it can be useful!

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