photoshop for astronomy session #1 – levels & curves astroimager’s special interest group...
TRANSCRIPT
PhotoShop for AstronomySession #1 – Levels & Curves
AstroImager’s Special Interest GroupApril 2007
Image Processing Workshop Outline
i. Levels & curves
ii. Sharpening & blurring
iii. Zone selection
iv. Star shaping
v. Color balancing
vi. Mixing Ha & color data
vii. PS add-ins for gradient & noise reduction
Workshop Goals
One-hour length with room for discussion
Files distributed in advance
PowerPoint file uploaded after meeting
Hands-on processing of basic process covered
“On-topic” discussion encouraged/needed
Suggested Resources
Zone System for Astro Imaging, by Ron Wodaski
PhotoShop Astronomy, by Scott Ireland
Making Every Pixel Count, by Adam Block.
Image Processing Workflow
File Preparation: Image calibration & combination, pixel cleanup
Basic Histogram Stretching – Levels & Curves
Smooth the Dim Zones
Sharpen the Bright Zones
Special Problems – Star Shaping, Gradients, Artifacts
Color Combination
Final Adjustments, Printing & Publishing
Wodaski’s Zone System
“Each image is broken down into multiple zones. By applying the appropriate type of processing to each zone, you can bring out the best in any image”
Dark zone – Noise is extreme & should be eliminated from imageDim zone – Significant noise but can preserve detailsMiddle zone – Noise is moderateBright zone – Noise is minimal
“The lower the noise level in any part of the image, the more sharpening you can perform. The higher the noise level, the more you need things like smoothing”
Definitions
Histogram – a graph that shows the relative number of pixels at each brightness value
Noise – random uncertainty in brightness values
System noise – repeatable noise that can be eliminated
Sharpening – image processing technique that empasizes details
Smoothing – image processing technique to reduce noise
Black point – sets the darkest value in the image
White point – sets the brightest value in the image
Definitions
Clipping – when a curve reaches the top line of the graph before reaching the right side of the graph
Curves – PS tool for making non-linear adjustments to the histogram – fixed number of levels so if you stretch one image section, you must contract another section
Levels – PS tool for making linear adjustments to histogram
Linear – adjustments that preserve internal brightness relationships
Non-linear – adjustments that do not preserve brightness relationships – part of image may get a boost in brightness where another may actually be dimmed
PS Processing Checklist – Basic Histogram Stretching
1 – Levels: set white point
2 – Levels – set initial black point
3 – Curves: use Standard Curve to expand dim and compress bright areas
4 – Levels: refine black point
5 – Repeat last two steps until you can see dimmest details
6 – Curves: determine the value of the dimmest visible details
7 – Curves: Boost dimmest details
8 – Curves: balance brightness levels between zones and make any inter-zone contrast adjustments
“The Standard Curve contains a point to brighten the Dim Zone; a point to establish the boundary between expansion and contraction and if necessary a Control Point to yield a flat line to the top right corner (eliminates clipping)”
Curve Point 1 – place where at dimmest area of image and use a steep slope to brighten Dim Zone aggressively
Curve Point 2 – place where there is visible but moderately bright detail and use a less aggressive slope to reduce the impact on the Middle Zone
Curve Point 3 – place at bright spot still not saturated and make the curve “flat to finish” to eliminate clipping
Unprocessed TIFF file showing little or no image detail
Steps #1 & #2 – Levels: set white point & initial black point
Curve Point 1 – place where at dimmest area of image and use a steep slope to brighten Dim Zone aggressively
Step 3 – Curves: use Standard Curve to expand dim and compress bright areas
Curve Point 2 – place where there is visible but moderately bright detail and use a less aggressive slope to reduce the impact on the Middle Zone
Curve Point 3 – place at bright spot still not saturated and make the curve “flat to finish” to eliminate clipping
Step 4 – Levels: refine black point
5 – Repeat last two steps until you can see dimmest details
5 – Repeat last two steps until you can see dimmest details – this can take several iterations and adjustments become finer
Other tools can adjust the final histogram but take care to prevent clipping
Brightness/Contrast command makes the same adjustment to every pixel in the image (linear adjustment)
Shadows/Highlights enables separate controls of the shadows and the highlights and is useful where astronomical targets have “burned out” areas
Step 8 – Curves: balance brightness levels between zones and make any inter-zone contrast adjustments
Where do you go from here?
Experiment with aggressiveness in settingsPost JPEGs on Yahoo for reviewFollow-up questions & discussion on groupTry other data files
Practice, practice, practice , practice, practice , practice, practice , practice, practice , practice,
practice , practice, practice , practice, practice , practice, practice , practice, practice , practice, practice , practice, practice , practice, practice , practice, practice , practice, practice , practice, practice, practice, practice , practice, practice , practice, practice , practice, practice , practice, practice , practice, practice , practice, practice , practice, practice , practice, practice , practice, practice , practice, practice, practice, practice , practice, practice , practice, practice , practice, practice , practice, practice , practice, practice , practice, practice , practice, practice , practice, practice , practice, practice , practice, practice, practice, practice , practice, practice , practice, practice , practice, practice , practice, practice , practice, practice , practice, practice , practice, practice , practice, practice , practice, practice , practice, practice, practice, practice , practice, practice , practice, practice , practice, practice , practice, practice , practice, practice , practice, practice , practice, practice , practice, practice , practice, practice , practice, practice, practice, practice , practice, practice , practice, practice , practice, practice , practice, practice , practice, practice , practice, practice , practice, practice , practice, practice , practice, practice , practice, practice
And practice some more!