Figure 29 shows the Blending Panel. It gives you options to control the color blending, alpha and depth for the current layer. You can also assign channels from other layers to be used as an alpha channel in the current layer.
Figure 29. Blending Panel
The Color Mode section allows you to choose from five different color blending modes. They are Normal, Add, Multiply, Screen and Difference. You can cycle through them with the Previous Mode and Next Mode buttons or you can right click and choose a mode from the context menu. The next slider controls the Layer opacity in percent. You can invert the colors by checking the Invert Color box.
Normal - This is the standard blend mode which simply takes each pixel from the top layer if present. Otherwise the bottom layer is used.
Add - This blend mode simply adds pixel values of one layer with the other. In case of values above 256 (in the case of RGB), white is displayed.
Multiply - Multiply blend mode multiplies the RGB numbers for each pixel of the top layer with the RRB number for the corresponding pixel from the bottom layer. As a simple multiplication can get values as high as 65025, which is far higher than the maximum allowed value, 255, the result is divided by 255. The result is a darker picture.
Screen - With Screen blend mode the values of the pixels in the two layers are inverted, multiplied, and then inverted again. This is in some way the opposite of multiply. The result is a brighter picture.
Difference - Difference subtracts the top layer from the bottom layer or the other way round, to always get a positive value. Blending with black produces no change, as values for all colors are equal to 0. (The RGB value for black is 0,0,0). Blending with white inverts the picture.
The Alpha Mode section controls how the Alpha channel is applied. Again you can choose from several modes - Ignore Alpha, Normal Alpha, Premultiplied Alpha and Premultiplied Mask. You can cycle through them with the Previous Mode and Next Mode buttons or you can right click and choose a mode from the context menu. You can invert the alpha channel by checking the Invert Alpha box. The different alpha modes will wield different results when applied. The following explanations assume a Normal blending mode.
Ignore Alpha -The image is treated as fully opaque. The existing alpha channel (if any) is ignored. The alpha channel of the result will show a white rectangle.
Normal Alpha - The foreground color is mixed with the background color according to the formula:
Code Block
foreground * alpha + background * ( 1 - alpha )
As an example, a 70% opaque red foreground - (R:1.0, G:0.0, B:0.0, A:0.7) - and a green background - (R:0.0, G:1.0, B:0.0) - will be combined to form the color (R:0.7, G:0.3, B:0.0).
This blend mode is suited for images with independent color and alpha channels. These images can be color-corrected without running into problems with their transparent portions (such as antialiased edges) no longer blending correctly.
Premultiplied Alpha - The first portion of blend formula above - foreground * alpha - does not depend on the background color, so it's possible to perform it beforehand. This is known as pre-multiplication - the image color is pre-multiplied by the alpha channel. Such images are mixed with the background according to the modified formula:
Code Block
foreground + background * (1 - alpha )
Since the foreground has already been multiplied by the alpha, this produces the same results as the "Normal Alpha" formula when applied to the original, non-premultiplied image data.
Getting back to our example, the premultiplied equivalent of our foreground color (R:1.0, G:0.0, B:0.0, A:0.7) will be (R:0.7, G:0.0, B:0.0, A:0.7) and the result of its composition over a background color (R:0.0, G:1.0, B:0.0) will again be (R:0.7, G:0.3, B:0.0).
If a non-premultiplied image is composed over a black background - a color of (0.0, 0.0, 0.0) - the Normal Alpha formula gives us:
Code Block
foreground * alpha + 0 * (1 - alpha )
which is the same as:
Code Block
foreground * alpha
In other words, the result of a non-premultiplied image composed over black is a premultiplied image.
The reverse operation - obtaining a non-premultiplied image from one that has already been premultiplied - can be accomplished in an image editing package by using the "Remove Black Matte" command.
(Note that the accepted terminology in this case - Normal Alpha and Premultiplied Alpha - is a bit confusing as it is the image color being premultiplied, not the image alpha.)
Premultiplied Mask - In a standard dynamic range image, the values of the color channels are between 0.0 and 1.0 (inclusive). This means that a premultiplied image would never have a color value bigger than the corresponding alpha value (because the maximum color value of 1.0, multiplied by an alpha value A, yields A).
Not all images conform to this rule, though. It is not uncommon to come across a premultiplied image in which some pixels have a color value bigger than the alpha value.
This may be intentional and can represent high dynamic range (color values higher than 1.0) or an additive glow (non-zero color values in areas where the alpha is zero).
Sometimes the image data is not intended to contain such color values. Using Premultiplied Alpha with such images can lead to compositing artifacts. In these cases, one can use the Premultiplied Mask mode, which is the same as Premultiplied Alpha, except that it clamps the color value to the alpha value, effectively constraining the colors into the safe 0.0 - 1.0 range.
The next two pictures illustrate the difference between Premultiplied Alpha and Premultiplied Mask.
Figure 30. Premultiplied Alpha and Premultiplied Mask
Figure 31. Image rendered with 3D software
The Depth Mode parameter allows you to choose from three blending modes for the depth channel. They are Ignore Depth, Replace Depth and Combine Depth. You can cycle through them with the Previous Mode and Next Mode buttons or you can right click and choose a mode from the context menu.
Opacity and Fade – the next three sliders control the Opacity and Fade effects for the current layer. The first controls the Opacity – you can drag it to change the opacity or right click and reset its value from the context menu. The next two sliders allow you to animate the Opacity of the layer in order to achieve Fade-in and Fade-out effects. The opacity will always be animated from 0% to its current value or vice versa. The left and right sliders control the length (in frames) of the Fade-in and Fade-out effects respectively. A value of 30 for the left slider means that the Opacity of the layer will be animated from 0% to its current value for 30 frames. The animation will start at the beginning of the sequence or at the In point if there is one. A value of 30 for the right slider means that the Opacity of the layer will be animated from its current value to 0% for 30 frames. The animation will start 30 frames before the end of the sequence or the Out point if there is one.
Pdplayer gives you the option to assign any channel from any layer in the scene to act as alpha channel for the current channel. This is done through the Alpha Source options. First you must select a layer to act as a source. You can do that by cycling through the available layers with the right and left arrow or by right clicking on the layer chooser and picking a layer from the Alpha Source context menu. Next you select a channel from the layer to be used as Alpha source. Again you can cycle through the different channels in the layer or right click on the channel chooser and select a channel from the Channel context menu. Next you have the option to choose whether or not to invert the source channel and if the two layers have different sides you can choose to stretch the source layer to fit the current one. Then you can choose a blending mode for the source channel. The available options are Replace, Combine with and Multiply with. Finally you can choose with which channel from the current layer you’d like the source channel to blend. You can choose either by cycling through the options within the layer chooser with the left and right arrow or by right clicking on it and choosing a channel from the Channel context menu.
