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Targets as Tools for Measuring Digital Image Quality
- 1. Image Science Associates Targets as Tools Not Talismans Don Williams - Image Science Associates d.williams@imagescienceassociates.com If you aim at nothing, you’ll hit it every time ZA – October, 2013
- 2. Image Science Associates 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 ZA – October, 2013
- 3. Image Science Associates Targets Physical references for measuring and monitoring imaging performance a requirement for disposable dog biscuit packages ZA – October, 2013
- 4. Image Science Associates 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 ZA – October, 2013
- 5. Image Science Associates 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 ZA – October, 2013
- 6. Image Science Associates 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 – ZA – October, 2013
- 7. Image Science Associates 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 ZA – October, 2013
- 8. Image Science Associates 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 ZA – October, 2013
- 9. Image Science Associates Digital Image Capture System - From light to numbers - The different ways an image is modified at capture light source sample Image forming optics sensor Processing Digital image file ZA – October, 2013 1. Introduction
- 10. Image Science Associates Targets For Benchmarking light source sample Image forming optics sensor Processing Digital image file Targets and Performance Analysis Software ZA – October, 2013
- 11. Image Science Associates How Targets Are Used to Measure and Manage Image Quality variation • Accuracy - Image values from a target are compared with established aim values. These values are typically usecase 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. ZA – October, 2013 bias
- 12. Image Science Associates - 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 ZA – October, 2013
- 13. Image Science Associates Important Image Quality metrics to monitor • Grayscale response • White Balance /Color • Resolution ZA – October, 2013
- 14. Correct Exposure via OECF ( tone transfer) OECF example 250 average green count value ( 8 bit) Image Science Associates 200 150 100 50 0 0 0.5 1 neutral (gray) density Density ZA – October, 2013 1.5 2
- 15. Image Science Associates 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 ZA – October, 2013
- 16. Image Science Associates 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 ZA – October, 2013
- 17. Image Science Associates 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 ZA – October, 2013
- 18. Image Science Associates Resolution Example 1 line = 1 space = 1/200“ 1 line = 1 space = 1/300“ 1 line = 1 space = 1/400“ Etc. ZA – October, 2013 Dpi scale
- 19. Image Science Associates # Pixel quantity is not image resolution: quantity vs. quality Limiting resolution = whenever all five lines are undetectable Resolution Sampling Frequency Though the sampling rate is increased to 600 dpi the true resolution for this scanner is only 300 dpi. ZA – October, 2013
- 20. Image Science Associates Performance Monitoring What could go wrong ? ZA – October, 2013
- 21. Image Science Associates What’s missing from this picture ? The different ways an image is modified at capture light source sample Image forming optics sensor Processing Digital image file 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. ZA – October, 2013 1. Introduction
- 22. Image Science Associates Session-to-Session Performance Using Device Target ZA – October, 2013
- 23. Image Science Associates Workflow Performance Monitoring Book Scanner Example ZA – October, 2013
- 24. Image Science Associates Workflow Performance Monitoring Book scanner example - ZA – October, 2013
- 26. Image Science Associates - Workflow Performance Monitoring keeping it simple with gray scales Book Scanner Example ZA – October, 2013
- 27. Image Science Associates - Workflow Performance Monitoring – OECF four-day holiday syndrome ZA – October, 2013
- 28. Image Science Associates 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 ZA – October, 2013
- 29. Image Science Associates End 1907 autochrome ZA – October, 2013