What is alpha channel?

Image processing (alpha channel, RGB, ...) ) Qilian (Adobe Series tutorial) Shcheyan UI Course

A question that may be silly, what is alpha in image processing?

the Alpha Channel is a term in computer graphics that refers to a special channel, meaning "non-colored" channels, which are primarily used to save selections and edit selections.

Why use ' Alpha ' to represent transparency?

L Alpha does not mean transparency, it does not represent clarity. Opacity and transparency are related to transparency, the former is opacity and the latter is transparency. For example, the "opacity:0.5" in CSS is that the set element has 50% opacity.

L each pixel of an image has RGB three channels, and later Alvy Ray Smith proposed to add an Alpha channel for each pixel, with a value of 0 to 1, to store whether the pixel has "contributed",0 to the image, and 1 represents opacity. " That is, the "alpha channel" stores a value, and its outward appearance is that "transparency",alpha and transparency are of no relevance.

Why I named the Alpha channel, I think because this is the "first channel" except RGB meaning, there is no other deeper meaning.

L"alpha Channel" is an intrinsic attribute of the image, which sets the transparency with CSS or other external methods, and does not change the value of the Alpha channel of the image.

Alpha channels (Αchannel or alpha channel) refer to the transparency and translucency of a picture. For example, a bitmap that uses 16 bits per pixel, for each pixel in the graph, may represent red in 5 bits, 5 bits for green, 5 bits for blue, and the last bit to be alpha. In this case, it either represents transparency or not, because Alfabite has only 0 or 12 different representations of possibilities. Also like a bitmap that uses 32 bits of storage, each 8 bits represents red, green, and blue, and alpha channels. In this case, not only can it be transparent or opaque, the alpha channel can also represent a level 256 semi-transparent because the alpha channel has 8 bits that can have 256 different data representation possibilities.

Other languages

RGBA is a color space that represents red (red) green (green) blue (blue) and Alpha. Although it is sometimes described as a color space, it is actually just an addition of additional information to the RGB model. The color used is RGB and can belong to any RGB color space, but Catmull and Smith presented this indispensable alpha value for 1971-1972 years, making alpha rendering and alpha compositing possible. The author uses Alpha to name it from the classical linear interpolation equation Αa + (1-α) B is the Greek alphabet.
Linear interpolation is a simple interpolation method widely used in mathematics, computer graphics and other fields.

What really makes a picture transparent is not that alpha is actually a numeric and other numeric value represented by Alpha.
Like you have a picture you want to pick out a part of the middle of the picture in PS you will create a mask and fill the mask with the unwanted place in black the required retention white this time actually did a multiplication
Use the value 0 represented by black to multiply the place you fill, and this place becomes transparent.

Photoshop channel classification, function, alpha channel, etc.

Channel

Channel as a part of the image, is inseparable from the format of the image, the color of the image, the format of the channel determines the number of channels and patterns, in the channel panel can be seen intuitively.

The channel is different, naturally we give them the name is different, below is their classification!

1. Alpha Channel

2. Color Channel

3. Compound Channel

4, Spot color channel

5. Vector Channel

Function article:

I. Definition of a Channel

Second, the function of the channel

Alpha Channel article:

Tool Operation Chapter

1. Using the selection tool

2. Using Drawing Tools

3. Using the Image Adjustment tool

4. Using Filter effects

What is a filter? From Wikipedia

Filters are often used for camera lenses as a color palette and for adding effects. such as UV mirror, polarizing mirror, Starlight Mirror, a variety of color filters. Filters are also used in the drawing software for manufacturing effect tools, known as Photoshop, for example, it has stylized, brush strokes, blur, twist, sharpen, video, sketch, texture, pixelated, render, artistic effects, and other

Alpha Channel
The Alpha channel is a specially designed channel for saving the selection area. When generating an image file, you do not have to produce an Alpha channel. Usually it is generated by people in the process of image processing, and read the selection of region information from. As a result, the Alpha channel is deleted when the output is not related to the resulting image. But sometimes, for example, in the final rendering of the output of the three-dimensional software, an Alpha channel is generated, which is used for post-synthesis in the plane processing software.

In addition to Photoshop's file format PSD, GIF and TIFF-formatted files can hold the Alpha channel. GIF files can also use the Alpha channel for image-to-background processing. As a result, we can use this feature of the GIF file to make shapes of any shape.

The application of alpha channel in the development of multimedia courseware works

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The application of alpha channel in the development of multimedia courseware works

Alpha channel Technology The most basic technology of image synthesis, its application is limited to the individual links in the development of multi-media courseware, and the alpha channel function is not given enough attention. such as: The general image processing software in Photoshop, the alpha channel and its application has a deeper understanding, and neglected the alpha channel in animation, video and multimedia integration tools. Through the comprehensive analysis of alpha channel, this paper aims to make the application of alpha channel technology in the development of multimedia courseware to form a relatively complete theoretical system, and play a specific guiding role in the development of multimedia courseware works.

First, the concept and function of alpha channel

In computer graphics, an RGB color model of the true Color graphics, used by the red, green, blue three color information channel synthesis, each channel with 8-bit color depth, a total of 24 bits, including all color information. To achieve the transparency of the graphics, take a method of adding another 8-bit information to the processing and storage of the graphics file, the additional channel information representing the transparency of each of the dots in the graph is called the alpha channel.

The alpha channel uses a 8-bit binary number to represent 256 levels of grayscale, or 256 levels of transparency. The alpha pixels of white (value 255) are used to define opaque color pixels, while alpha-channel pixels of Black (value 0) are used to define transparent pixels, and alpha pixels of grayscale (value 30-255) between black and white are used to define different degrees of semitransparent pixels. As a result, a graphics card with an alpha channel is displayed on a 32-bit bus, which can result in a transparent or translucent visual effect.

A mathematical model of a transparent or translucent graphic should be as follows:

To facilitate the analysis below, set the alpha value [0,255] interval map to the corresponding value representation of the [0,1] interval, which is the value between the alpha value 0-1. The individual pixel points in the drawing file can be represented as:

Graphx (Redx,greenx,bulex,alphax)

The display value of the corresponding pixel on the screen is converted to:

Dispx (Redx*alphax,greenx*alphax,bluex*alphax)

The alpha channel is not only used for transparent or translucent display of a single graphic, but more importantly, it is widely used in image synthesis.

Here is an algorithm for image blending based on alpha channel data:

In fact, we calculate the original RGB components that do not contain the alpha component and then add the colors that need to be combined. Such as: Two images are A and b, by the combination of these two images of the image is called C, you can use the following four-tuple to represent the figure A and B, ternary group represents the image C:

A: (RA,GA,BA,ALPHAA)

B: (Rb,gb,bb,alphab)

C: (RC,GC,BC)

According to the above algorithm, the following:

Rc=ra*alphaa+rb*alphab

Gc=ga*alphaa+gb*alphab

Bc=ba*alphaa+bb*alphab

This is the component of the three primary colors after mixing the two images. If there are multiple images that need to be mixed, mix them according to the method two two.

The application of alpha channel in the preparation of multimedia materials

1. Alpha channel in Photoshop for image processing software

In the image processing software Photoshop, the channel is the most basic concept, the color channel represents the main color information of the image, the additional channel has a spot color for printing channels and storage, modify the selection area of the alpha channel.

Typically, a new channel created separately is the alpha channel, which does not store the color of the image, but instead stores the selection field as a 8-bit grayscale image and is added to the image's color channel. Thus the content of the alpha channel represents not the color of the image, but the selection area, where the white represents a fully selected area, Black is a non-selected area, different levels of grayscale represent different selection percentages, up to 256 levels of gray scale.

The operation of the alpha channel content is to create, store, and modify the selection area we need. Any level of selection can be achieved if the target layer is uploaded into the selection (that is, the alpha channel is applied). A number of special effects can be produced by the calculation between multiple alpha channels or the composition of the alpha channel and layer. As a result, the alpha channel function can be described as a great magic weapon for Photoshop to edit the background images, theme images, buttons, navigation signs and other materials used in the composition multimedia works.

2. The application of alpha channel in animation authoring software

The alpha channel is widely used in both the two-dimensional animation software and the three-dimensional animation software.

The well-known 3DMAX animation software provides a working environment (i.e., video post) for the post-synthesis of animations, which can be used to process many different levels of image files and animation scene compositing at once using video Post's image compositing function. The most frequent synthesis of alpha composing is achieved by the alpha channel.

3DMAX can produce 32-bit file formats (especially 32-bit Targa image files) that contain Alpha channels based on the output format of the file. When rendering, any area without an object has an alpha value of 0, a solid object with an alpha value of 255, and a transparent material with an alpha value between 0~255. This feature of 3DMAX enables a variety of transparent overlapping image or animation effects in the software or other software.

Flash Vector Animation also has the use of alpha channel function. Alpha is a major attribute of the graphical symbol (graphic) in the Flash animation scene, changing its value (between 0~100) to change the degree of transparency of the object symbol. If a graphic symbol is set to a different alpha value at each keyframe, the graphic symbol exhibits a dynamically changing transparency effect. By writing an action script (actionscripting), the user interactively changes the alpha value to make the animation work more lively and interesting.

3. Transparent synthesis of high-stack orbits using alpha channel in video editing software

In video processing software, it is common to overlap multiple video clips As required to create a transparent or translucent effect. Up to 97 video high-stack tracks are available in premiere movie editing software for the synthesis of image fragments. There is a transparent setting option for compositing in the video option command for transparency, which provides a number of key type Basic (transparent) types. Where alpha channel key is used to create a transparent effect on the selected area of the fragment using the image or the alpha channels of the image.

Some film and television creators in the program editor, in order to put the advertising icon, report spent maths and other patterns as markers or column headings superimposed on the screen, using Photoshop, 3DMAX or Cool3d and other software to generate a 32-bit TGA picture or picture sequence with alpha channel, The alpha channel key function is used in non-woven system software to achieve a high stack of title captions or icons.

The application of alpha channel in Multimedia integration tool Authorware

Authorware5.0 images in previous versions are available in only five transparent ways: Opaque mode (opaque), matted mode (edge transparent mode), Transparent mode (transparent mode), inverse mode (reverse mode), Erase mode (erase modes). The common problem with these transparent methods is that only the transparent white part is pure, and the translucent image can not be achieved. Authorware5.0 and later versions already support the alpha channel as a transparent way to picture. Using the alpha channel of the image, the information is precisely transparent to each part of the screen, producing a perfectly transparent or translucent effect. In particular, semi-transparent images done with other tools are still, and the parts in the picture are fixed, while translucent compositions implemented in Authorware with Alpha transparent mode combined with transitions transition properties and position motion animation settings can be dynamically performed, Not only can arbitrarily change the position of the parts, but also can arbitrarily adjust the size of each part, the flexibility of its operation and the richness of the effect is conceivable.

The author has used the alpha channel technique in the teaching courseware of using Authorware5.0 to imitate Chinese characters ' writing strokes, and achieved good results. Take the word "wood" as an example.

The first step, using Photoshop5.0 to do the following four pictures, respectively named "Wood 1.psd", "Wood 2.psd", "Wood 3.psd" and "Wood 4.psd." The four pictures are exactly the same, except that the alpha channel is different.

The second step is to load four images into the Authorware5.0 document in the order shown below:

In the third step, all display icons are alpha mode transparent to render the corresponding strokes, and the appropriate transition transitions are set for each display icon to show the correct writing process for each stroke. Icon "Wood 1.psd" Select the Transition property is wipe in the wipe right, the icon "Wood 2.psd" Select Wipe down in wipe, the icon "Wood 3.psd" selected Stirps strips in Top,build left, Icon "Wood 4.psd" select strips on top,build right in strips. To make the effect more accurate, you can add a wait icon between the icons to ensure a certain interval between strokes or to control the writing speed by clicking the mouse.

Iv. concluding remarks

The alpha channel technology itself is not complex, but its use of creativity is limitless. It is a never-ending topic to summarize the ways of using alpha channel technology in the development of multimedia courseware to improve the technical quality of works and enhance the artistic appeal of works.

In OpenGL, the alpha value usually works after the blending operation has been activated. Used to combine the processed slice color values with the pixel color values stored in the frame cache. Without a blending operation, each new slice will be considered opaque by OpenGL, overwriting any color values that already exist. The use of hybrid operations allows for translucency and other states. you can look at the alpha value this way, consider the RGB component of the slice as a representation of the color, and consider the Alpha value component as the opacity of the color.

What is alpha channel?