Each layer in a window can have the opacity of that layer individually set from zero opacity to 100 percent opaque. Layers that are partially transparent will allow objects, labels and pixels from layers underneath them to be partially visible. Opacity is controlled by settings in the Layers pane.
The illustration above shows a map with two layers, a vector drawing layer of buildings shown above an image layer from a Bing satellite image server. Both layers are at 100% opacity, so the upper, buildings layer does not allow any part of the layer below it to show through the area objects used to show buildings in that drawing.
To change the opacity of a layer in a window, open the window and then choose the Layers pane. Double-click into the % opacity value for the layer and change the number to whatever level of opacity is desired, and press Enter to accept the new value.
For example, we can double-click into the opacity value for the buildings layer and change it from 100% to 40%.
The result is that the buildings layer becomes about 40% opaque, that is, it blocks about 40% of the visibility of layers below, allowing the image layer to be partially visible through the area objects that represent buildings.
100% opacity (default) means the contents of the layer are opaque. 0% opacity means completely transparent: the contents of the layer will be invisible, because they are totally transparent. When entering opacity numbers, there is no need to enter the % percent character: simply enter a number from 0 to 100 and press ENTER.
Layer opacity will be combined with whatever opacity is already defined within the component. For example, RGBA images may have per-pixel transparency enabled via the A alpha channel. Using layer opacity with such an image layer will apply the layer opacity throughout the entire RGBA image in addition to any per-pixel transparency.
Changing the opacity of a layer does not make any changes to the component involved. It only changes how that component is displayed in the layer stack in that particular window. If we have an RGBA image participating as a layer in a window, when we change the opacity for that layer there is no change to the alpha channel values for each pixel, and there will be no change in the opacity value of layers in other windows if that image also participates as a layer in other windows.
Layer opacity works with any layer that may be displayed in a window.
Labels layers, for example, may also be made partially transparent in windows as shown above.
In the Layers pane we have set opacity for the Labels layer to 40%.
Layer opacity works just as transparency does when stacking layers of transparent plastic film. The results are visually combined and are not simply arithmetically additive.
Suppose we have a map with three drawing layers, each of which contains a blue square. The screen shot above shows the window. Two of the layers have layer opacity set to 50% and in these two layers the blue squares have been positioned to overlap each other. The third layer has 100% percent opacity. The Background has been turned off so we see the checkerboard pattern indicated when there is no background color.
As expected, the square in the layer with 100% opacity appears completely opaque. The checkerboard background is not visible through it at all. The squares in the partially opaque layers have been rendered partially transparent so that the checkerboard background may be seen through them. Where the squares overlap the background is less visible because it is seen through two layers of partially transparent "material."
If we turn on the default, white background in the Layers pane we can see how the objects appear against a white background. This illustrates an effect that is occasionally confusing: the color of the region where both squares overlap is lighter (because of opacity) than the completely opaque square. We may be surprised by that if we think that if one square has 50% opacity and the other square has 50% opacity then a sight line through both squares should be 100% opaque.
In reality, opacity is more a percentage multiplication effect than addition. When see through one square that is 50% transparent the sight line results in combined color that is 50% from the square and 50% from the background. If we now place another 50% filter of the same color into that sight line the result is color that is 75% the color of the squares and 25% the color of the background. Thus, an overlap of two filters each of which is 50% opaque has the same effect as one filter with 75% opacity and is not the same as 100% opacity.
Layer opacity is a property of a Map layer and not of the drawing or other component that is shown in that layer. The same drawing, for example, can appear as a layer in more than one map and in each map the layer containing the drawing can have a different opacity setting. The opacity used within each map for that layer appears in the mfd_meta table as a Property of that map.
Changing the opacity of a layer does not make any change to the component shown in that layer. Layer opacity simply changes how the window shows the component in that layer.
Transparency and opacity are two terms that mean the same concept viewed from different directions. When something is completely opaque it is not at all transparent. When something is perfectly transparent it may be said to have zero percent opacity.
Which word is used depends on the discussion. When imagining layers stacked up above each other like transparent sheets it might be more natural to use the word transparency. When discussing a specific percentage of light transmission to be applied in a dialog most applications use the word opacity.
The convention in the graphics arts editing software industry is to adjust layer opacity as a number from 0% to 100% opacity, so that an image with 100% will be fully opaque and will not allow any view of an image underneath it. Manifold follows this convention.
User Interface Basics