Invisible Pixels and Selection

By default, selection methods do not select regions of invisible pixels. The one exception is Touch Selection, which will select invisible pixels when they are touched.




Suppose we delete part of our standard bronze image to create an image with invisible pictures. We've also drawn a blue bar across parts of the image to divide it into upper and lower halves. We have turned on the Border in the Layers pane so we can see the actual size of the image.




If we click near the lower right corner with Touch Select, all the invisible pixels contiguous to the spot touched will be selected. Note that the contiguous selection "flows" only through the actual extent of the image. Therefore, it did not select "around" the blue band or the lower leg of the monument.




If we SHIFT-click with Touch Select we can select all of the invisible pixels in the image no matter where they are located.


Other Selection Commands


All other selection commands will ignore invisible pixels by default.


Use the SHIFT key to Select Invisible Pixels


Select Touch will select contiguous invisible pixels when the mouse is clicked on a region of invisible pixels. All other mouse selection commands, such as Select Box, will select only visible pixels within the mouse selection region. To select invisible pixels as well, hold down the SHIFT key while making the mouse selection.


A SHIFT - Select Touch command will select all invisible pixels in the image, whether contiguous or not.



A Select Circle selection does not select the invisible pixels. It only selects visible pixels within the selection circle.



Using SHIFT - Select Circle selects both invisible and visible pixels within the selection circle.


Invisible Pixels Saved Selection


Precisely because invisible pixels are not seen there are times when we might like some help in seeing them.


Invisible pixels are very frequently used when composing sophisticated images (that is, maps) that consist of stacks of many constituent image components. When working with many different image components that contain transparent regions we might forget which parts are transparent and which regions are simply white pixels in images or "white space" in drawings seen through the layer stack.


When working in an image window we can see which regions are invisible pixels by working with the background color turned off to allow the checkerboard pattern to show through. However, in the case of RGBa images it is possible to set RGBa pixel transparency to full transparency to make the pixels entirely disappear. The effect is the same, but these are not invisible pixels. They are visible pixels with their alpha values set to full transparency for each individual pixel and so the checkerboard pattern will be seen through such pixels as well.


In addition, we often would like to use the region of invisible pixels in selection combinations with selections made in other parts of the image. To support this usage and to make it easy to unambiguously see the region of invisible pixels Manifold devotes one of the seven saved selections in images to a system-provided saved selection that always shows the region of invisible pixels. This saved selection will appear in the Selections pane if any invisible pixels occur in the image.




Suppose we have an image that may contain invisible pixels.




images\btn_preview.gif To see the region of invisible pixels, simply click on this saved selection to highlight it and press in the Preview button in the Selections pane toolbar.




Any invisible pixels in the image will appear in blue preview color.




This example shows use both of the invisible pixels saved selection as well as the use of the SHIFT key to select invisible pixels in addition to visible pixels.




We will use the globe.bmp sample image with contrast increased (to make touch selection easy in water areas) and relief added for a pretty appearance.




Using SHIFT - Touch Select we touch regions with water to select them throughout the image.




Choose Edit - Delete and the pixels disappear. (We have the white background color turned on in the Layers pane so the background appears white).




However, the invisible pixels are still there as placeholders for the region they occupy. We can click on a region of invisible pixels with Touch Select and they will be selected. They are simply invisible. Once selected we can pour color into them with the paint bucket to make them visible again.




Selections made over invisible pixels do not normally select them. Here we have made a Select Circle selection that ignores the invisible pixels.




To select invisible pixels, we hold the SHIFT key down while making the selection. For example, a SHIFT - Select Circle command will select the invisible pixels within the circle as well as the visible pixels.




It's often the case that we would like to see the region of invisible pixels or use it with selection combinations. Clicking the invisible pixels saved selection in the View - Selections pane will show it to us in blue preview color if the Preview button is pushed in.




We can use the invisible pixels selection like any other saved selection. For example, we can use Subtract from Selection to subtract the region of selected pixels from the circle selection.




If we push out the Preview button to remove preview color we can see that we have just made a circular selection within Africa that is "clipped" at the coastline. This is a somewhat contrived example since we could have done the same thing just by selecting the circle without holding down the SHIFT key as we did in an earlier step. However, there will be many times when it will be convenient to use the invisible pixels saved selection in Replace, Add, Subtract, Invert or Intersect commands together with the existing selection.