skintone steganography for real time images technicalseminor topic briefly.

skintone technical seminar topic

At a very, very basic level, this is how common photographic film works: film is thin plastic covered in a layer or layers of photosensitive chemicals. When exposed to a very small amount of light — like the tiny amount that hits it when a photographer presses the camera’s shutter button — a subtle image, invisible to the naked eye, forms on the film. Developing the film makes the image visible (as a negative — the parts that were exposed to the most light are the darkest, this is inverted in the printing process).

Color film is coated with many layers of chemicals, with chemicals keyed to different colors in the different layers. The earliest color photographs were actually made up of three separate black and white photographs, one taken through a red color filter, one taken through a green color filter, and one taken through a blue color filter. Color filters do just what it sounds like -- they filter light by color. A red filter only let red light through, resulting in a photo that was brightest where the source reflected the most red light. The three black and white photos were then combined either by projecting colored light through them and fine-tuning the composite; or by turning them into single-colored cells which could be stacked, illuminated, and manipulated in a device called a chromoscope; or by mechanically turning them into colored prints. This process was first proposed in 1855.

Color film was invented quite a bit later. The principle was basically the same: take impressions of different wavelengths of light as separate photographs, and combine them into a full-color photo. The difference was they were able to take these “separate” photos in the different layers of chemicals on the same square of film; essentially a single photograph, and a single exposure (press-of-the-button.)

The proportion of the different colors in the photo -- the photo’s color balance -- was partially determined by the concentrations of chemicals in the film; and partially by the various chemicals used in processing the film and printing from it. This underwent a lot of fine-tuning. For example: the first color film was two-color, i.e. only in red and green. In the US, where this technology was being pioneered, most of this fine-tuning was done by affluent white men. And, as it turns out, the race of these men had implications: they were operating under a very particular assumption about whom and what their product was meant to capture.