CSC/FAR 020, Computer Graphics, September 2-3, 2014 Dr. Dale E. 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 / PHL resolution.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 / D90field zip to your Desktop f22-25, f and f 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 … = 0 … x 2 x 2 x 2 x 2 x 2 x 2 x 2 = 2 8 = 256 combinations 256 degrees of red x 256 degrees of green x 256 degrees of blue = 2 24 = 16,277,216. Fewer pixels give fewer, more discrete (less continuous) color gamuts.

Color Modes RGB is for each of Red, Green and Blue, based on 8 bit resolution of color data CMYK is % for each of Cyan, Magenta, Yellow and Black pigments / printer ink. HSB (hue-saturation-brightness) is 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.