targets as tools, not talismans - don williams
TRANSCRIPT
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Image Science Associates
ZA – October, 2013
Targets as Tools
Don Williams - Image Science Associates -
Not Talismans
If you aim at nothing, you’ll hit it every time
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Image Science Associates
ZA – October, 2013
What makes cultural heritage imaging different ?
- Future Re-purposing
- Variety of use cases
- Information durability
- Information critical
- Images, not pictures
- Scientific and
research component
- High volume, High speed
- Think manufacturing
- Think error management
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Image Science Associates
ZA – October, 2013
Targets Physical references for measuring and
monitoring imaging performance
a requirement for disposable dog biscuit packages
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Image Science Associates
ZA – October, 2013
Target Examples
• Intended for both calibration, performance testing, and quality control.
• Act as an archeological reference for light and resolution values. Required for change detection.
• Things to consider:
- Do they suit your purpose ?
- How comprehensive are they ?
- Are they self described ?
- Reflection or Transmission
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Image Science Associates
ZA – October, 2013
Why Use Targets ?
• Quality control and industry compliance
• Consistent product
• Verify vendor’s claims
• Managing expectations – acceptance testing
• Accurate Metadata population
• Diagnostics – problem solving and image processing identification. Less rework
• Effective communication
• Corrective actions
Used by both US and Netherlands Initiatives
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Image Science Associates
ZA – October, 2013
The Top Ten FourTips
1 – Targets as Tools, Not Talismans
2 – Get the exposure correct
3 – Keep the Neutrals neutral
4 – Don’t confuse Pixels with Resolution
5 –
6 –
7 –
8 –
9 –
10 –
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Image Science Associates
ZA – October, 2013
What You’ll see
• Standardized ways to measure scanner/camera performance
• Identify sources of variability in digital imaging
• Introduce imaging quality control procedures into workflows.
• Use easy, non-disruptive ways to monitor performance
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Image Science Associates
ZA – October, 2013
What is an image ? ….and how is it measured ?
- A two dimensional spatial structure of varying light levels. It is characterized by measuring a camera or
scanner’s response to light levels over a two
dimensional space. These levels are often classified into colors types ( i.e. RGB)
- Changes in light can occur over short distances, like
edges,( high frequencies) or larger distances or areas,
like sky or facial features( low frequencies).
Targets act as known input references by which
a camera/scanner’s output can be compared.
SPACE & LIGHT
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Image Science Associates
ZA – October, 2013
Digital Image Capture System - From light to numbers -
The different ways an image is modified at capture
Digital image file
light source
Image forming optics
sensor sample Processing
1. Introduction
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Image Science Associates
ZA – October, 2013
Targets For Benchmarking
light source
Image forming optics
sensor sample Processing
Digital image file
Targets and Performance Analysis Software
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Image Science Associates
ZA – October, 2013
How Targets Are Used to Measure and Manage Image Quality
• Accuracy - Image values from a target are compared with established aim values. These values are typically use-case dependent.
• Precision - Tolerances around these aims are also provided. Small tolerances
imply greater precision but also higher production costs. The opposite is true for
large tolerances.
Consistent performance is often more important than accuracy.
variation
bias
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Image Science Associates
ZA – October, 2013
- Targets – Good measurements require good targets
Target Elements 1) ISO Frequency Response (SFR) and resolution
over the field of view. (7” x 10”)
2) Human interpretable resolution features
3) Dimensional scales for sampling confirmation
4) Automated feature detection
5) Neutral gray uniform background
6) Ten spectrally neutral gray patches
7) Self described colorimetric patch annotations
Device Target Object Target
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Image Science Associates
ZA – October, 2013
Important Image Quality metrics to monitor
• Grayscale response
• White Balance /Color • Resolution
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Image Science Associates
ZA – October, 2013
Correct Exposure via OECF ( tone transfer)
Density
OECF example
0
50
100
150
200
250
0 0.5 1 1.5 2 neutral (gray) density
aver
age
gree
n co
unt v
alue
( 8
bit)
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Image Science Associates
ZA – October, 2013
White Balance - Keep all of the neutrals, neutral -
OECFs can be measured for each color channel using a target’s neutral gray patches.
85% of good color imaging performance is keeping the Red, Green, and Blue OECFs the same… Really!
This is a good example of a well white balanced capture
Note that all color channel OECFs lie on top of each other
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Image Science Associates
ZA – October, 2013
White Balance
This is an example where the white balance performance is marginal.
Note how the blue channel OECF departs from the red and green OECF
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Image Science Associates
ZA – October, 2013
White Balance Assessment Photoshop Histograms
Though gray patches appear gray, the one patch is actually slightly colored.
All three histogram should align if white balance is correct
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Image Science Associates
ZA – October, 2013
Resolution Example
Dpi scale
1 line = 1 space = 1/200“
1 line = 1 space = 1/300“
1 line = 1 space = 1/400“
Etc.
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Image Science Associates
ZA – October, 2013
# Pixel quantity is not image resolution: quantity vs. quality
Limiting resolution = whenever all five lines are undetectable
Though the sampling rate is increased to 600 dpi the true resolution for this scanner is only 300 dpi.
Resolution
Sampling Frequency
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Image Science Associates
ZA – October, 2013
Performance Monitoring
What could go wrong ?
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Image Science Associates
ZA – October, 2013
What’s missing from this picture ?
The performance of a digital capture system at the sensor and beyond is influenced by all of the above in
addition to operator training and environment.
The different ways an image is modified at capture
Digital image file
light source
Image forming optics
sensor sample Processing
1. Introduction
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Image Science Associates
ZA – October, 2013
Session-to-Session Performance Using Device Target
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Image Science Associates
ZA – October, 2013
Workflow Performance Monitoring
Book Scanner Example
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Image Science Associates
ZA – October, 2013
Workflow Performance Monitoring
Book scanner example
-
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Image Science Associates
ZA – October, 2013
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Image Science Associates
ZA – October, 2013
- Workflow Performance Monitoring keeping it simple with gray scales
Book Scanner Example
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Image Science Associates
ZA – October, 2013
- Workflow Performance Monitoring – OECF four-day holiday syndrome
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Image Science Associates
ZA – October, 2013
How to Start
• Identify a small pilot project – Equipment Benchmarking ? – Workflow Monitoring ?
• Choose an appropriate target – Device target – Object target
• Practice introducing targets – Will it remain in place ? – Technique, not results – How often ?
• Don’t do too much – Do not analyze data – Small amounts of data
• Collect data
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Image Science Associates
ZA – October, 2013
End
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