Pixel processing (DRAFT, topic is treated in global background)

Pixel processing

  graph TD
     PV(pixel values) --> PO(processing operation)
     PO -.- |e.g.| SV(subtract value)
     PO --> |replaces| PV(pixel values)

  graph TD
     PV("pixel values") --> MO("processing operation")
     MO --> NPV("new image")

  graph TD
     PO("processing operation") --> |changes| PV(pixel values)
     DT("data type") --> |limits| PV

Activity: Pixel based background subtraction

• Open image: xy_8bit__nuclei_noisy_different_intensity.tif
• Appreciate the significant background intensity
• Measure pixel values at [ 28, 35 ] and [ 28, 39 ]
• Measure the image background intensity in this region:
  • upper left corner at [ 20, 35 ]
  • width = 10
  • height = 10
• Subtract the measured background intensity from each pixel.
• Measure the pixel values again.
• Observe that the resuls are incorrect.

Repeat above activity, but:

• After opening the image, convert its data type to floating point.

Activity: Explore the limitations of float data type

• Create an empty image
• Set all pixel values to 1000000000.0
• Add 1.0 to all pixel values
• Be shocked…

…it turns out that from 16777216 on you cannot represent all integers anymore within a float.

Formative Assessment

True or false?

• Subtracting 100 from 50 in a 8-bit image will result in -50.
• Adding 1 to 255 in a 8-bit image will result in 256.
• Subtracting 10.1 from 10.0 in a float image will result in -0.1
• Adding 1.0 to 255.0 in a float image will result in 256.0
• Adding 1000.0 to 1000000000.0 in a float image will result in 1000001000.0

Learn more

• Limitations of float