Images and the Human Eye
Fun Fact:
- The Human eye perceives no difference between the quality of a photo taken by a digital camera from an analog camera at 3.2 MP. This is not because of the margin error between the two pictures, this is because of our perception. In reality the margin error between the two photos could be even larger than 5%. That’s why in digital image processing (dip) it’s very common to use perception tests. A perception test consists of showing a group of random people a bunch of images, so that they can tell you if the images are visible or not. The results of a perception test might depend on the kind of research you’re doing
Image Compression
- Compressing an image might imply losing information. This depends on the compression algorithms used. For example, WhatsApp doesnt have great image compression algorithms, so if you need to preserve as much information you might want to use an alternative compression software before sending images via WhatsApp. Relations between Pixels In the image is from the GIMP software, an image manipulation program. In the red boxes I’ve highlighted 300 ppp and pixel/in. PPP is short for puntos por pulgada It’s Spanish for dots per inch or dpi. The size pixels are not standardized, it mostly depends on your application and what image you want to digitize.
- For example, say that you made it as a successful engineer and you work at NASA. Now imagine that you want to take an image of a specific area on the surface of mars to make a map. It would be highly useful to know the representation or the equivalence of a pixel on real metrics. Depending on context, a pixel could represent 10 cm or 1 mm or 1 m; this is entirely up to the application, but it certainly is something important to think about in the field of digital image processing.
Color
- You may know that we humans perceive color via the light that bounces off of certain materials. We perceive color because of some structures in our eyes called cones; there are three types of cones. We have red, blue, and green cones in which each cone reacts to red, blue, and green light respectively
- This is the reason why pixels represent these colors. This is known as the RGB system.
- Along the course we will continue to study about colored pictures but for now, let’s determine how many colors we can represent if each we have 8 bits per channel. Three channels means 8x3 bits or 24 bits, which means 2²⁴ = 16777216 or 16 MB colors so around 16 million colors.
Thought experiment
How do panoramic images work? Say that you take two pictures of a given space in panoramic mode; the algorithms compare the images and join them when the algorithm detects that the edges are the same. However, there’s no secret that us humans can’t operate with mechanical precision. So when we want to take a sideways panoramic photo, we have to consider that our arm will not only move sideways but also slightly upwards and downwards. To account for this we use math operations such as translation, rotation, and interpolation to have a complete and clean panoramic photo.
Elements of visual Perception
To continue studying about digital image processing it’s also important to know the basics of how humans perceive images.
- Our eyes, 20 mm in diameter approx
- Is enclosed by 3 membranes
- cornea and sclera outer cover:
- The cornea is a tough, transparent tissue that covers the anterior surface of the eye. Continuous with the cornea, the sclera is an opaque membrane that encloses the rest of the optic globe
choroid
- It lies below the Sclera. This membrane contains a network of blood vessels that serves as a major source of nutrition to the eye. Even superficial injury to the choroid can lead to severe eye damage as a result of inflammation that restricts blood flow. Iris: controls the amount of light that enters the eye ~2–8 mm
retina
- Cones(6–7 million) and rods(75–150 million)
- Cones:
- Fovea: a circular indentation in the retina of about 1.5 mm in diameter.
- Photopic bright-light vision 150,000 elements per mm2
- The number of cones in the region of highest acuity in the eye is about 337,000 elements.
- Rods: Overall picture of the field of view Sensitive to low levels of illumination Scotopic dim-light version.
note: A charge-coupled device (CCD) imaging chip of medium resolution can have this number of elements in a receptor ray no larger than 5 mm x 5 mm.
- Our eyes actually capture images and because of the lenses it inverts everything that we see. It’s our brain that actually helps us piece together the images and see things as they actually are, not upside down.
Brightness Adaptation and Discrimination
- In DIP, formulas for error are not used too often. This is because our eyes are not too sensitive to small changes, that’s why
- we use perception tests. We also use the Weber ratio or Weber’s Law, this helps us gain reference in quantifying the perception of change in a given stimulus.
