Digital Imaging

Week 6 Part I: Math with Pixels

Lesson Objectives

After this lesson, students will be able to:

  • view how digital colors work in terms of basic mathematic functions
  • describe the basic technology behind blending colors and image processing
  • apply traditional darkroom tools to create various image manipulations
  • create a multi-layered blended image
  • investigate various filters in Photoshop


This week we will take a look at blending modes and filters that you can use for making adjustments to an image or combining images in interesting ways and learn a little bit about the math going on behind the scenes to make it all happen.


Colors you see on your computer and other light emitting devices, like cellphones and TVs use an “Additive Color Model’. On the other hand, colors on surfaces don’t emit light, like paper and house paints, can be described through the “Subtractive Color Model’. Adobe Photoshop and many other imaging software applications use the Additive Color Model by default.


Blending Modes

In this section, we will use various Photoshop “Color Blending’ modes to layer and blend the colors of pixels. This operation is based on very simple math, and you can find similar operations available in other applications, such as video editing software.

Blending modes work with brightness and darkness values from 0 to 255 in Photoshop. Photoshop standardizes these values before doing the math for blending. When values are standardized, White (normally RGB 255) becomes 1 and Black (normally 0) remains at 0.

The ‘blend layer’ is the layer upon which the blending mode is applied. The ‘base layer’ is the layer underneath, that remains unchanged.

Blending modes will Darken, Lighten, Overlay, show Difference, or change the Hue. Within each category, there are multiple blend modes to choose from. To put it simply, Photoshop is multiplying between the background base layer and the foreground blend layer.

Multiply a color (RGB) from the base layer (R=230, G=24, B=12) by white (1) = the original color because x*1=x. So, if you use a Multiply blending mode any area of white, you will get the base layer color. And, the areas of the dark color will remain dark x*0=0.

normal blend mode multiply blend mode

Let’s look at a second example. Here, a dark image on a lighter background. The Multiply blend mode gives us dark values where the dark is multiplied with the underlying colors (0) and the lighter area colors are blended with the base layer. An additive blend mode (remember additive RGB color mode) appears lighter because x+1=(a number closer to white).

normal color modemultiply color modeadditive color mode

For a much deeper explanation of blending modes, see Blending Modes Explained.


Read about filters from Adobe.

In addition to the Filter Gallery, play with some of the ‘Other’ filters available from the Top Menu -> Filter -> Other area. These filters are often overlooked but are interesting. High Pass is essentially a sharpening filter, for example.

other filters


Further Reference


Week 6 Part I (A & B)



Select one image you like, and copy paste other images on top of the first image as layers. Try using 3-7 images or parts of images.

Use Layer Blending Modes for each layer and mix colors of pixels and experiment with blending modes you have not used before. This is pretty open but the goal is to play around, move layers up and down with blending modes to create something visually interesting.

Look for interesting colors and effects (instead of realistic images). This exercise is NOT about creating “coherent realistic’ images. Strange effects like crazy unexpected colors, jugged images and high contrast brightness are OK. However, try to create a visually appealing image with the blending results. You can also mask areas away from layers if desired.

Set the image resolution to 800-1000 pixels on the larger length of the image and save your image without flattening the layers as Photoshop(.psd) and share with your instructor over Google Drive.   Save a JPEG version for posting the image to the class site.


Open an image of your choice different from those you used on A. Before applying filters, set the image size to 800-1000 pixels (width or height, which ever the greater) (Image menu -> Image size).

Experiment with  the Filter Gallery as well as following filters  (available under Filter menu-> Others):

  • Minimum/Maximum
  • Offset
  • High Pass
  • Custom

The goal of this part is to play specifically with the Filter Gallery to create something interesting with just one photo.

Save your image without flattening the layers as Photoshop(.psd) and share with your instructor over Google Drive.   Save a JPEG version for posting the image to the class site.


  1. Create a new post on the site with the category ‘Week 6 Part I’.
  2. Upload the images you worked on for Exercise 7 A & B.
  3. Journal as follows:
    • Part 1:  What blending modes did you use on this image and how many layers?
    • Part 2:  Write an explanation on what you did to the image, including the name of filters. You should use only Minimum, Maximum, Offset, High Pass and Custom filters.

Discussion info is in Week 6, Part II.