Remember you can edit whole Rectangles.
Brush Parameters are mentioned in several places below; they are set on the dialog that appears when you press 'B'. The shape of the Brush is a square of from 1 to 51 Pixels, the centre of which is at the tip of the Mouse pointer. Brush Size is the number of pixels from the centre to the middle of any edge: a value of 0 gives a 1 Pixel brush, 1 gives a 3 pixel square brush, 2 gives a 5 Pixel Brush, etc. The Brush Delta is a measure of how closely a Pixel's colour should match that of the Pixel under the Mouse Pointer before it is used. Note that it is quite possible to Paint etc. on both sides of a narrow feature if they are within the width of the brush and the colours match closely enough.
The Shift key is often used in conjunction with the Left Mouse Button or the letter 'C' to obtain the reverse effect. Hold down Shift while pressing/dragging the button or pressing the letter. You can put Shift Lock on instead, but remember to turn it off again when no longer needed.
Fixed Pixels will not be altered by any of these operations.
There are two modes of operation for the Brush: Painting and Fixing. The current state is indicated on the Title Line; change it by pressing 'F'.
When you use the Mouse Pointer as a Brush you must be in Magnified View. If you start in Full View, just point at the upper left corner of the region you wish to edit, hold down the Left Mouse Button, drag a rectangle around the required area, and release the button. A dotted rectangle will appear. If you press the Right Mouse Button, this rectangle will be magnified and you will be able to edit its pixels. This procedure will fail if the selected rectangle is too big; the original pixels can only be shown at a one to one scale or greater.
It is even easier to magnify a rectangle when in Normal View (reached by repeatedly pressing the Right Mouse Button). Just drag out a rectangle as above, but this time as soon as you release the Left button, Magnified view is reached directly. You are guaranteed to be able to magnify an area from Normal View because the pixels are already one to one. There are a couple of other advantages gained from starting in Normal View: right click returns to the same area after editing, and you can use the Home, End, Page Up and Page Down keys to roam around in Normal View and quickly switch to Magnified View when you spot a repair that needs to be done, then swap back and continue roaming from where you left off.
You can perform these operations when Painting appears on the Title Line and you are in Magnified View. Point the mouse at a single Pixel and click the Left Button without moving the mouse, and a single pixel will change. Do the same holding the button down and dragging, and you will draw with the current Brush Parameters.
Deleting simply makes the corresponding Pixels in the Picture Empty. This operation is performed by holding the Shift Key down.
Painting can be performed by pointing at the Active Frame or the Picture. In either case pixels are transferred from the Active Frame to the Picture. The Brush Delta is applied to the visible Frame or Picture, thus you can easily replace a uniform area by drawing on the Picture, or transform a similar area from a Frame.
You can perform these operations when Fix is on the Title Line and you are in Magnified view. In many ways these are similar to the last operations, Fix without Shift and Unfix with. You cannot Fix Empty Pixels. Use F to make Fixed Pixels visible. In Magnified View, if the scale is large enough they are shown with a dot in the upper left corner, otherwise as a solid white/black area. If they are hard to see, press Space which inverts the colour.
You can perform these two operations when in any View. Just point at an area and press C or shift-C. The colour of the Pixel under the Mouse Pointer is noted, and pixels are transferred from the Active Frame to all locations on the picture, or deleted from locations, which match those in the test area whose colours are similar to the one noted, how similar is controlled by Brush Delta. Sounds complicated but in practice it easy to use and very useful. Unlike Painting this operation will not cross a boundary, but be careful it can leak through the tiniest gap. Fix Pixels to prevent this. Set Undo Point before starting so you can Undo in case of leaks.
Suppose you want to remove a fairly uniform background from a Picture. Point anywhere in it, hold down Shift and press C. If there are isolated areas, repeat pointing at those. Suppose you want to be sure the background comes from the back frame. Point at areas on it and press C. If you would like to substitute the Active Frame in place of another Frame in a given place in the Picture, then view the Depthmap, point at the area to replace and press C. To fill an Empty area of a Picture from the Active Frame point at it and press C. I'm sure there are many other uses.