CSC/FAR 020, Computer
Graphics, September 3-4, 2013
Dr. Dale E. Parson
http://faculty.kutztown.edu/parson
Outline for class # 3
Image Information
• You start with only as much information as you
capture with the original photo or scan.
• Brightness, contrast, resolution, focus, depth of field.
• You cannot add information concerning the original
image with a software tool.
• You can subtract information concerning the original
image with a software tool.
• You can add new information via a software tool.
• Transform the image in Photoshop, not the camera,
unless capture-time transforms help you to see.
Resolution
• A pixel is a picture element.
• Each pixel is a conceptual set of sensors, e.g., one
sensor per red, green and blue.
• The number of virtual sensors = pixel width x height.
• How much resolution in bits at each sensor?
• Analog light amplitude (intensity) and frequency (color)
translate to bit-level descriptions via quantization.
• The fundamental dimensions of a digital image are in
pixels (width x height), not inches or centimeters.
Compute pixels per inch (ppi) based on image size.
Resolution examples
• Copy and extract Parsons / Outbox /
PHL20090824resolution.zip onto Desktop.
• Inspect this image folder in Adobe Bridge.
• Slides #402 through #408 vary pixel dimensions while
keeping all other parameters roughly constant.
• File type is JPEG, a compressed image format that
uses variable rate lossy compression to reduce file
size and transfer time.
Resolution examples
• Inspect metadata for #408 in Adobe Bridge.
• f / 4.6 – aperture – the smaller the denominator, the
bigger the aperture (more light), less depth of field.
• 1 / 250 – shutter speed – the smaller the denominator,
the slower the speed. Slower than 1 / 60 can blur.
• ISO 64 – “film speed” equates to sensor amplification –
high speeds are better for dim light or fast action (faster
film), but they are grainier due to amplification noise.
• Focal length of 6.3 mm (35 mm focal length of 38.0
mm) – a higher number corresponds to optical zoom in.
Comparing #408 (3264 x 2448) to
#407 (1920 x 1080) for resolution
• Using Command + and Command – and Command 0,
zoom in until you can count the number of pixels
dedicated to the down arrow below Terminal F. How
many do they have?
• Try the same with # 405 (640 x 480).
• Use Command 1 to view actual pixels – one pixel
from the image mapped to one screen pixel. Does
the resolution to your eye follow from the captured
resolution?
• More resolution means more data to store and transmit.
Downsampling a copy of #408 to
compare to #407
• Close All in Photoshop, Open #408, and save it as
Copy408 in PSD (Photoshop) format.
• Close 408 and Open 407 and Copy408.
• On 407 use Image -> Image Size to determine pixel
and other dimensions.
•
On Copy408 check “Resample Image,” change to pixel
dimensions of 407 (1920 x nochange), and set resampling
algorithm to Bicubic Sharper.
• Use Command 1 and Command + to count pixels and compare
down arrow on Terminal F. Which shows more detail?
• Downsampling loses information even on the pixels it retains!
Upsampling a copy of #407 to
compare to #408
• Close All in Photoshop, Open #407, and save it as
Copy407 in PSD (Photoshop) format.
• Close 407 and Open 408 and Copy407.
• On 408 use Image -> Image Size to determine pixel
and other dimensions.
•
On Copy407 check “Resample Image,” change to pixel
dimensions of 408 (3264 x nochange), and set resampling
algorithm to Bicubic Sharper.
• Use Command 1 and Command + to count pixels and compare
down arrow on Terminal F. Which shows more detail?
• Upsampling “makes up” (interpolates) approximate information.
Optical versus digital focal length
(zoom)
• #408 through #412 increase focal length from 6.3mm
to 18.9 mm using optics (by repositioning the lens
and refocusing).
• #413 and #414 use “digital zoom” to spread out the
pixels. There is no new optical info.
• Compare #410 (14.3 mm) and #412 (18.9 mm) for
resolution by zooming into Terminal 15 arrow.
• Compare #412 (18.9 mm) and #413 (49 mm digital)
to see if digital zoom increases resolution. What do
you see?
Compression
• Photoshop uses lossless compression to make
files smaller. No pixel information is lost.
• JPEG uses lossy compression. It throws away
pixel information that “mortals can’t see,” or
at least that are not important to a photo’s
use. JPEG supports different levels of
compression. (MP3 files are lossy audio files.)
• Compare #382 and #399. Both have 3264 x 2448 pixels,
but #398 uses 2.05 Mb while #399 uses 1.27 Mb.
Shutter speed, aperture and depth
of field
• Copy and extract Parsons / Outbox /
D90field20090826.zip to your Desktop
• f22-25, f10-125 and f4.5-500 gives increasing
aperture / shutter speed pairings of f / 22 and 1 / 25,
f / 10 and 1 / 125 and f / 4.5 and 1 / 500 respectively.
• As the aperture increases, so does the shutter speed.
• The depth of field decreases.
• What differences do you see in these images.
– Watch out for the shakes with slow shutter speeds!
• Both JPEG and RAW file formats are present.
Moving back and the zooming in
on a foreground figure
• Compare focal length of 18 mm to 105 mm for
the same foreground figure.
• Doubling distance to foreground figure halves its size.
• The new distance is not doubled to remote background
figures. It is only a small fractional increase.
• Doubling the focal length now doubles the apparent
size of all figures in the frame, not just the foreground
figure.
• What else changes in the composition?
• Show size of an uncompressed TIFF file.
Color Modes and Histograms
• Close All files, then set background to black.
• New File, RGB Color, black background.
• Add ovals in Red, Green and Blue.
• Add ovals in the pairwise sums of these. Add all 3.
• RGP is additive. It applies to light-adding displays.
• New File, CMYK white background.
• Fill in combinations of CMYK? Do they look the same? (C+M=blue,
M+Y=red, Y+C=green).
• Cyan-Magenta-Yellow-Black is subtractive.
• It applies to pigment mixing, high quality printing.
• Some dot matrix printers use RGB.
8 bits of Red, Green and Blue in
RGB
•
•
•
•
•
Each pixel has 8 bits of resolution.
Each bit can store a 0 or 1.
00000000 … 11111111 = 0 … 255
2 x 2 x 2 x 2 x 2 x 2 x 2 x 2 = 28 = 256 combinations
256 degrees of red x 256 degrees of green x 256
degrees of blue = 224 = 16,277,216.
• Fewer pixels give fewer, more discrete (less
continuous) color gamuts.
Color Modes
• RGB is 0 .. 255 for each of Red, Green and Blue,
based on 8 bit resolution of color data
• CMYK is 0 .. 100% for each of Cyan, Magenta, Yellow
and Black pigments / printer ink.
• HSB (hue-saturation-brightness) is 0 .. 360 degrees
for position in a color wheel, saturation is % distance
from center of color, and brightness is % brightness
value.
• L a b is a device-independent mode with a large
gamut.
Color Layers and Histograms
• Explore the layers of the RGB and CYMK files.
• Investigate the color histograms of the 18mm
and 105 mm pictures of the fields and the
stop sign.
• Compare dusk lake shots of various degrees of
underexposure and defocus.
• Sometimes suggestions are better than accurate data.