Alpha and index color transparency

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"In a transparent background, I cannot see white in the preview. As long as I change to a transparent color, all the white will become transparent. If I set it to opacity, objects with the same background color will become white. "-- M
I would like to answer this question by following me in an experiment. Downloading this lab image will save you a lot of work.
If you haven't downloaded the file, start by opening a new file with a white canvas. The left half is covered with a black rectangle. Draw a Red Square 2/3 in the size of the canvas and place it in the middle of the black and white canvas. Finally, draw a small white rectangle. Copy it and change the color to black. Move them so that the white squares reside in the black part of the background, and the black squares are placed in the alignment with the white background.

Select File> export preview ). Select index transparency from the bottom left ). The white canvas and rectangle will become transparent. [Note: it is very important not to use droppers to select colors.] If you change the option to no transparency, all white colors are restored.

Finally, select Alpha transparency ). The white on the canvas disappears, while the white square is retained. Click Cancel to exit the export Preview (export Preview ).

Select modify canvas color ). Change the white background to black and the black to white. Similarly, the color inside the red box is reversed.

As you did just now, go to the export Preview (export Preview) and select index transparency ). The black background disappears. Then select no transparency ).

This time, when you select Alpha transparency, the black square inside will remain unchanged.

Index color is the canvas color, that is, the background color. The Alpha value ranges from 0 (black) to 255 (white). Black indicates transparency, and white indicates opacity. If the white part of your image is attached to a certain color, even if the background is white, after using alpha transparency, the white part on your image is still opaque.
Since the black color based on Alpha chromatography basically represents transparency, why is the black rectangle in the middle of the image still retained in the previous example? I guess it's becauseProgramI don't know why I can't see it through its attached color. Maybe some experts can give us an authoritative answer, so I will be very happy to announce it here.
At the same time, I want to say, Macromedia, do not modify it!