Editing effects within Manifold are applied only to those channels that are checked ON in the View - Panes - Layers pane. Painting commands will also be applied only to those channels that are checked ON. If you have not done so, please read the Images and Channels topic and the RGBa Pixel Transparency topic before proceeding.
Let's begin by filling an image with black color so items we draw will be clearly visible. If we draw a triangle using white color it will appear white with all three R, G, B channels ON. White paint is a color that has maximum values in Red, Green and Blue.
If we check OFF the Red and Blue channels and check ON the Green channel the triangle appears green because we only see the green values of the pixels.
Checking the Red channel ON and the other channels OFF shows only the red color values in the white triangle.
If we continue to use white as the color of our painting tool and add three dots with only the Red channel ON, they will appear in red color.
If we switch ON all three RGB channels we see the white triangle appears in white again but the dots drawn using white paint color are still red. Because the other two channels were OFF when the dots were drawn their pixels were created using only the Red channel values.
We can now click the Red channel OFF and the Blue and Green channels ON. The triangle appears cyan in color because cyan is the color obtained from an even mix of Blue and Green with no Red. Note that the three small dots are not visible because their pixels have no color values in the Blue or Green channels. If we draw a large dot using white paint it will be created in cyan color because the blue and green values from the white paint color will be written into the pixels using the active Blue and Green channels.
We now click OFF the Green channel so that only the Blue channel is ON and then we create a very large dot using white paint color. It appears blue because only the blue values from the white paint are written into the active Blue channel. The triangle appears blue and the other dot appears blue as well. We see only the blue part of the (blue + green) cyan color for the cyan dot. We do not see the three small dots because they do not have any blue values in their pixels.
If we click ON all three channels we can see the image as it has been created. All pixels were painted using white color. Only the white triangle was painted with all three channels ON so only it is white. The red dots were painted when only the Red channel was ON so they only have red color values. The large blue dot was painted when only the Blue channel was ON so the pixels in the large blue dot only have blue values. The pixels in the cyan dot were painted when both the Blue and the Green channels were ON so they have both blue and green values.
Painting into the Alpha Channel
When painting into an image's R, G and B channels the color values for each channel come from the separate R, G and B values of the paint color being used. When painting into the Alpha channel the value for each pixel comes from the intensity of the paint. Although any color paint may be used it is usually easiest to reckon the intensity by using different shades of gray. Because true gray colors have equal mixes of R, G and B color values the intensity of any shade of gray is simply the overall appearance of lightness or darkness of the gray color.
Let's take a moment to convert our RGB image into an RGBa image using the Image - Convert To command. We will now see an Alpha channel in the Layers pane in addition to the Red, Green and Blue channels. If we click the Alpha channel ON and all the other channels OFF we can paint into the Alpha channel without altering the other channels. We can paint a series of dots into the Alpha channel using a lighter shade of gray color for each dot. The result is a series of dots that have increasing amounts of transparency (the lighter the gray color, the greater its intensity and thus the greater the transparency).
If we click ON all four channels we can see how the differing transparency levels affect the image. In particular, we can take a close look at the blue dot, the cyan dot and the uppermost red dot to see how parts of the dots are now semi-transparent as set by the alpha transparency.
If we layer the above image in a map above our sample bronze image we can see how the transparency applied by painting different shades of gray dots into the Alpha channel now allows more or less of the bronze image to be seen through the upper image. Note how the blue, cyan and uppermost red dots are partially transparent in regions where the gray dots were painted into the Alpha channel.
See Also:
Combining Channels into Images