Machine Vision
How do you load signed 16 bit png file outside of labVIEW?
Solved!
I have a program that saves a signed 16 bit png file that I need to be able to load outside of labVIEW.
Looking through the documentation for IMAQ write file 2
it looks like either the bias (if bit depth is false) or the MSB info should be stored in the PNG header.
I've tried loading the meta data in python but I'm not seeing any data related to either of these values.
Where exactly in the header are these pieces of data stored and what are they called?
Thanks!
My recommendation is to stay away from signed PNGs and use only unsigned TIFFs if you would like to load the images using Python, OpenCV, ImageJ, etc
Let me explain. I will create simple two-pixels images, both signed and unsigned stored in PNG and TIFF:
Now I will load all four images into Python using OpenCV and check the range:
This is my code:
import cv2 print('Test Signed PNG - expected -1000...1000') image = cv2.imread('TestSigned.png', cv2.IMREAD_UNCHANGED) min_val,max_val, min_indx, max_indx=cv2.minMaxLoc(image) print(min_val, max_val) print('Test Unsigined PNG - expected 0...1000') image = cv2.imread('TestUnsigned.png', cv2.IMREAD_UNCHANGED) min_val,max_val, min_indx, max_indx=cv2.minMaxLoc(image) print(min_val, max_val) print('Test Signed TIFF - expected -1000...1000') image = cv2.imread('TestSigned.tif', cv2.IMREAD_UNCHANGED) min_val,max_val, min_indx, max_indx=cv2.minMaxLoc(image) print(min_val, max_val) print('Test Unsigned TIFF - expected 0...1000') image = cv2.imread('TestUnsigned.tif', cv2.IMREAD_UNCHANGED) min_val,max_val, min_indx, max_indx=cv2.minMaxLoc(image) print(min_val, max_val)
and this is result:
Test Signed PNG - expected -1000...1000
0.0 64031.0
Test Unsigined PNG - expected 0...1000
0.0 64062.0
Test Signed TIFF - expected -1000...1000
31768.0 33768.0
Test Unsigned TIFF - expected 0...1000
0.0 1000.0
Obviously the only unsigned TIFF is correct.
The problem could located in the encoder (NI Vision) or decoder (OpenCV) side.
Especially for PNGs I see in the hex dump that the internal NI Image Type is present:
This is how NI can recognize correct type when the image loaded back using IMAQ Read File.
And "Use bit depth" flag will not help much.
So, the easiest way is to use unsigned TIFF when 16 bit images needs to be loaded somewhere outside NI Vision.
