CSC/FAR 020, Computer Graphics, September 4-5, 2012

1. CSC/FAR 020, Computer Graphics, September 4-5, 2012 Dr. Dale E. Parson http://faculty.kutztown.edu/parson Outline for class # 3

2. 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.

3. Resolution • A pixel is a pictureelement. • 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.

4. 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.

5. 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.

6. 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.

7. 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!

8. 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.

9. 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?

10. Compression • Photoshop uses losslesscompression to make files smaller. No pixel information is lost. • JPEG uses lossycompression. 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.

11. 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.

12. 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.

13. 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.

14. 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.

15. 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.

16. 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.