What are Layers

The standard explanation of layers in image-editing is “think of layers as like a pile of acetates” or “layers are like transparencies laid on top of each other”.

The problem with this description is that anyone under 30 (I guess) is unlikely to have encountered the type of overhead projector (OHP) which uses transparencies.  In fact, the last time someone asked me if I knew where to find an OHP for an elderly presenter whose material was all on transparencies, my honest answer was “Try the museum”.

Understanding the overhead-projectors / transparency / acetate answer

An overhead project was a light box with a lens mounted above it which rotated light from the light-box by 90 or so degrees (using mirrors), to allow it to be projected onto a wall or screen

A transparency was an A4-size sheet of rigid plastic, made out of a chemical called acetate, and able to cope with the high temperatures generated by the bulbs in early overhead projectors.
So it was possible to use the projector to show text or an image from a transparency onto a wall or screen. They were fantastic pieces of technology in their day: they freed teachers from writing the same stuff on blackboards over and over again, they gave business people and trainers far more options for explaining things to groups of people, and they freed church-goers from being stuck to the same old hymns from their hymn book. But they were expensive: acetate transparencies cost a lot, as did the special pens that were needed to write on them. Eventually there were transparencies which could be put through laser printers, but the printed text tended to chip off them over time. as laptops got cheaper and data-projectors became more widely available.

A better explanation of layers

Yes, you can even get them in
pretty coloured edgings like this

Think of layers as similar to plastic page-protectors – the high-quality ones that you can see through properly, not the cheaper ones that look a little fuzzy.

And think of a file in your image-editing program as being like a ring-binder folder.

Now imagine putting a pile of page-protectors in a folder, putting some parts of an image into each of them

If you look into the pile from the above, then the picture that you see is the sum of all the picture-parts in the individual page-protectors:

• If there is a part of the page which isn’t covered up by the contents of any page-protector, then that part will look empty, AKA transparent.
• If one page protector has something which covers the whole page, then you won’t be able to see anything from page-protectors which are underneath that one. So most likely you want to put that “background” page-protector at the very bottom of the pile.

And that is how layers work in pretty much all image-editing programs (Photoshop, GIMP, Paint.net, etc)

Of course you can do more with layers in a computer programme than you can with page-protectors: you can be certain that items which you put into a layer will stay exactly where you put around and not wiggle out of place if someone moved the folder. But you can also you can make them semi-opaque, re-arrange the order of the layers using drag-and-drop, apply a colour filter to an entire layer, etc.

But the general idea is the same.


Next: Working with layers in Photoshop

What is digital imaging

Wikipedia says that

Digital imaging or digital image acquisition is the creation of photographic images, such as of a physical scene or of the interior structure of an object. The term is often assumed to imply or include the processing, compression, storage, printing, and display of such images.

The ECDL Foundation, who prescribe the European Computer Driving Licence curriculum, have used the phrase image-editing to mean working with images using image editing software (eg Photoshop).   Their curriculum (see my notes on it) doesn't include creating or capturing digital images, presumably because this is regarded as photography. 

Some local institutions which teach this image-editing curriculum use Digital Imaging as the title of their courses.

As the lines between computers and cameras has blurred (to the point where all cameras are now computers, and most computers and mobile phones include a camera), the distinction between image capture and image editing has become less useful.

The working definition in this blog is that Digital Imaging is working with digital images in any way:

• creating them, 
• editing or manipulating them, 
• outputting (on any medium - via a printer, plotter, screen)
• managing them  (storing, filing / cataloging, sharing, archiving, deleting).

Photoshop - basic image actions

Before doing any action in Phtotshop, you need to select the correct layer to work on.

How to select a layer

1. Use the Select drop down menu and choose all layer OR

2. Click on them in the layers tab.



Once you have selected a layer, you need to choose which part of the image on that layer to work on

How to select a whole image

• Use a selection tool: Click once to select; click again to deselect  OR
• Use a keyboard shortcut: the fastest whole-image selection, is Ctrl+A (Windows) or command+A (Mac).  OR
•  Use menu command: Selection > Select All (or similar - may chancge between versions)

Note: other useful items on the Selection menu: Select None or Deselect (Ctrl/d) that turns off an existing selection.

Use one of these tools to select part of an image:

• Lasso
• Polygonal Lasso
• Magnetic Lasso
• Elliptical Marquee
• Magic Wand
• (and more?)

Note that once you have selected the tool, you can see its options bar - tools have different options.

Properties of the Selection tools 

The Lasso, Polygonal Lasso, Magnetic Lasso tools are hidden in the Toolbox under one and the same icon. The icon on the Toolbox displays the last selected tool. Use the floating menu to choose one of these others. To access it, right-click on the arrow in the lower right corner of the displayed icon.

The Lasso tool 

This is useful for drawing freeform segments of a selection Select the Lasso tool, and set feathering and anti-aliasing in the options bar. (See Soften the edges of selections.), and then use the mouse to draw the border of the area.

The options bar has selectors for a new selection, or

• add to,
• subtract from,
• intersect with

the existing selection.


With the Lasso tool, you can:

• Drag to draw a freehand selection border.
• Switch between freehand and straight-edged selection by pressing Alt (Windows) or Option (Mac).
• Click where segments should begin and end.
• Erase recently-drawn straight segments by pressing the Delete key.
• Close the selection border by releasing the mouse without holding down Alt (Windows) or Option (Mac).
• Optional - Click Refine Edge to adjust the selection boundary.

To use the Lasso tool

• Select the tool
• Click at the first point of the area to be selected using the left button
• Left-click at points around the area, to outline it.

The Rectangular marquee tool

This is used to select rectangular and square areas.

• Activate it by clicking on the icon or select it from the floating window.
• Put the mouse cursor where the corner of an imaginary rectangular should be, and press the left button.
• Keeping the left button pressed, move the cursor to the diagonally-opposite corner
• Release the button.

To select a square area keep the SHIFT key pressed while selecting. But remember that this will also add the new selection to any existing selection.

The Elliptical marquee tool 

This is used to select ellipses and circles.

• Select the Elliptical marquee tool from the Toolbox by clicking on the icon or select it from the floating window.
• Put the cursor at the point where the corner of an imaginary rectangular with an inscribed ellipse should be
• Press the left button.
• Keeping the left button pressed, move the cursor to the diagonally-opposite corner
• Release the button.

Select a circular area by keeping the SHIFT key pressed. But remember that this will also add the new selection to any existing selection.

Alternative: hold the Alt (Windows) or Option (Mac) key while when selecting an elliptical or a rectangular area, and the selection is made from the center to borders, instead of corner to corner.

The Polygonal Lasso 

This is also used to make freehand selections, but the contour of the selections is made up of straight segments. To use it:

• Select the Polygonal Lasso tool
• Put the cursor on any point that is to be part of the outline,
• Click the left mouse button, to mark the first point of the contour.
• Move the cursor an adjoining point of the contour and left-click it: a straight-line selection will be made between the two points.
• Repeat the previous step until the 2nd-to-last point is made.
• Click on the first point (which is also the last point) again to close the contour.

The Magnetic Lasso 
This is also used to make a freehand selection, but you don't have to follow the contour of the object precisely: any object that stands out against the background of the selected area is traced automatically when you move the cursor along the object. To use it:

• Select the Magnetic Lasso tool
• Put the cursor on any point on the edge of the object to be selected
• Hold down the left button and start dragging the cursor along the object. Fastening points will appear as you outline the object and when you make a click. If a one is irrelevant, remove it by pressing the DELETE key and return to the previous fastening point to continue outlining the object.
• Close the contour by joining the first fastening point to the last one by bringing the cursor to the first point or by making a double-click.

The Magic Wand tool 

This elects a consistently coloured area.  The options bar has a place to set the Tolerance - for this, the higher the tolerance, the more colours will be selected with a single click. Valid values are 0 to 255:  0 means only one colour is selected, 2255 means all colours in the image will be selected.

To use it:
1. Select the magic wand tool.
2. Move the cursor to a pixel which must be in the selection and left-click. 
When you do this, an outline ("marching ants") appears around the pixel, and all other pixels in image with a similar colour, within the specified Tolerance value.

These selection tools are efficient due to the flexibility of their usage: you can add to, subtract from or intersect a selection.

• Add by holding the SHIFT key.
• subtract by holding the Alt (Option in Macintosh) key 
• Intersect by holding the SHIFT and Alt (Shift and Option in Macintosh) keys 

 while selecting more pixels with the "same" colour.

Invert a selection

Inverting a selection swaps the currenly selected and unselected areas  (so currently unselected becomes selected)

Use it from the menus, by choosing Select > Inverse.

Hint:  this is the easiest way to select an object which is set against a solid-colored area:  just select the solid colour with the Magic Wand tool, and the use Select > Inverse.

Softening the edges of a selection

There are two techniques for this.

Anti-aliasing smooths the jagged edges of a selection by softening the colour transition between edge pixels and background pixels. Only the edge pixels change, so no detail is lost. This technique is useful when cutting, copying, and pasting selections to create composite images.

Anti-aliasing is available for these tools:
Lasso
Polygonal Lasso
Magnetic Lasso
Elliptical Marquee
Magic Wand


You need to specify anti-aliasing before using the selection tool -  you cannot add it after selection.   Therefore to use it:

1. Select the tool.
2. Select Anti-aliased in the options bar.
3. Select the area to be anti-aliased.

Feathering blurs edges by building a transition boundary between the selection and the surrounding pixels. This may cause some loss of detail at the edge of the selection. 

Feathering is available for these tools:

• Marquee
• Lasso
• Polygonal Lasso
• Magnetic Lasso tool

Feathering effects only show after you move, cut, copy, or fill a selection.   So to use feathering

1. Select a tool.
2. Enter a Feather value in the options bar. (from 0 to 250 pixels)
3. Select the area to be feathered
4. CHECK - IS THIS WHERE YOU NEED TO CUT/COPY AND THEN PASTE?

To define a feathered edge for an existing selection
1. Choose Select > Modify > Feather.
2. Enter a value for the Feather Radius, and click OK.

Issues with feathering:
A small selection made with a large feather radius may be so faint that its edges are invisible and therefore not selectable. If you see the message “No pixels are more than 50% selected,” either decrease the feather radius or increase the size of the selection. Or click OK to accept the mask at its current setting and create a selection in which you cannot see the edges.

Effects (fx) vs Filters

An effect is applied to a selected item   (eg text, shape).

A filter is applied to a whole image.

Basic imaging concepts

A pixel (picture-element) is a minute area of illumination on a display screen. Many pixels can be put together to make a digital image.

Resolution is a measure of the number of pixels in an image. It is may be quoted as

• the width and height of the image, measured in pixels
• the total number of pixels in the image
• the total number of pixels per unit in the image.

For example, an image that is 2048 pixels wide and 1536 pixels high (2048 X 1536) contains (multiply) 3,145,728 pixels (or 3.1 Megapixels)

Colour concepts

Colour model

Colour model or colour mode is a way to define colour: it describes how colour will appear on the computer screen or when printed.

Some colour models are:
RGB
HSB
CYMK
Greyscale

The RGB (Red, Green, Blue) colour model

This colour model is used for colours that will show on TV screens or computer monitors.

A value between 0 and 255 is assigned to each of the colours – Red, Green and Blue - and the resulting colour is made up of the combination of them.

For a purely blue colour:  Red is 0, Green is 0 and Blue is 255 (pure blue).
For Black: Red, Green and Blue are 0
For White, Red, Green and Blue are all 255 (white light is made up of all colours on the spectrum, black the absence of them all).

The HSB-Hue, Saturation, Brightness colour model

Hue 

The the "basic colour" of something, measured as an angular value on the colour wheel.

Saturation 

This measures the purity of the colour, also called  chromaticity, especially in TV and video work.
A yellow that can't get any yellower is fully saturated.
To desaturate a color add grey.  Or think of it as subtracting colour and leaving grey behind (use which-ever description is more intuitive).

Brightness 

This is determined by how much black is mixed with the colour.
Colors aren't all perceived as being the same brightness, even when they are at full saturation, so the term can be misleading. This on it as the z-axis of a colour cylinder.

CMYK-Cyan, Magenta, Yellow and Black

This colour model is used for print work. It describes colours according to percentages of Cyan, Magenta, Yellow and Black.

Commercial printers uses these four colours to reproduce full colour artwork in magazines, books and brochures. By combining them on paper in varying percentages, the illusion of lots of colours is created.

Grayscale

In Grayscale mode an image is made up from different shades of gray in an image.  Every pixel has a brightness value ranging from 0 (black) to 255 (white) - so in an 8‑bit greyscale  image, there can be up to 256 shades of gray.

In 16-and 32‑bit images, the number of shades in an image is much greater than in 8‑bit images.

Grayscale values can also be measured as percentages of black ink coverage (0% is equal to white, 100% to black).




See this video for a view on how the colour models are different in what they can represent:

Colour depth

This is the number of bits which are allocated to store the value of a colour which is displayed.  The more bits per pixel, the more colours that potentially can be displayed  Each bit has a value of 0 or 1, so one bit can represent two colours.   N bits can represent 2^N colours so:

1 bit = 2 colours
4 bit = 16 colours   (= 2x2x2x2)
8 bit  = 256 colours
24 bit = 16,777,216 colours

Printing equipment today cna usually handle up to 8 bit colour - display screens can go far higher.

Colour Balance

This characteristic of a multi-colour image has a high value when the relative density (luminance - should this be luminescence) of all colours in the image is about the same and no one colour dominates.

Contrast, Brightness and Gamma

Brightness

Traditional brightness brightens the entire image from the shadows to the highlights equally.

Gamma is an alternative, non-linear, method of adjusting brightness.  It works by changing the mid-tones of the image only, not the highlights. Increasing Gamma can make an image look brighter, but it does so by increasing the brightness of the shadows and mid-tones, not the highlights.

Contrast

Contrast is the separation between the darkest and brightest areas of a image.

Increasing contrast increases this separation, so shadows are darker and highlights are brighter.

Decreasing contrast brings the shadows up and takes the highlights down, so they become closer to one another.

Adding contrast usually adds "pop" and makes an image look more vibrant while decreasing contrast can make an image look duller.

Transparency

In Photoshop, all layers, except Background layers, are transparent, ie have transparency capabilities.

Even if a layer is filled with pixels, eg with a full size image, or a fill colour, the layer itself has a transparency value, and if this is set to a high value, or if  pixels on the top layer are erased or masked, it allows other layers below to show through.

You can give any layer a degree of transparency by adjusting its opacity:   high opacity = low transparency, ie they are the inverse of each other.

If necessary, the background layer can be converted to a regular transparent layer.

Types of graphics: raster vs vector

Raster

An image made of lots (hundreds, thousands or millions) of tiny pieces of colour information. The pieces may be either pixels (colour blocks on an electronic screen) or dots ("blobs" of ink on a printed item).

Some raster file types:

• jpg (jpeg)
• psd - Photoshop File
• png
• tiff (tagged image file format)
• bmp
• gif

Also - TO BE CONFIRMED

• PSP – Paint Shop Pro File
• XCF – Gimp File
• CPT – Corel Photo Paint

Vector

An image drawn using points, lines and mathematical curves.

• A 1” x 1” square raster image a at 300 dpi has 300 individuals pieces of information
• A similar vector image only has four points (one for each corner) and the instruction that they form a square; software that processes vector images uses maths to connect the dots correctly.

Usually used in logos (which may need to be printed in very different sizes) and where a sharper finish is required.

Some vector file types:

• AI – Adobe Illustrator File
• EPS – Encapsulated Post Script
• PDF – Portable Document Format
• PSD – Photoshop File   (yes, Photoshop can work with both raster and vector formats)
• SVG – Scalable Vector Graphic
• CDR – Corel Draw File
• PLT – Plot File