How to edit transparency pixels in a FilterPlugin?

Question

I want develop a fiter plugin of photoshop. while i am not clearly about some issues:
(1) how does the photoshop provide the transparency data ( blank data ) to me ?
in other words, i want obtain the "alpha" channel data, is this provided by the "inData" together with other channels?
(2) I want to change the pixel's transparency, means i want change totally transparent pixels into
opaque. how to do it ?
(3) in the pluginName.r file，there are 7 entries in FilterCaseInfo like following. is there anything i must pay special attention to?
  FilterCaseInfo
    /* Flat data, no selection */
    inWhiteMat, outWhiteMat,
    doNotWriteOutsideSelection,
    filtersLayerMasks,
    worksWithBlankData,
    copySourceToDestination,
By the way, When i invoke a filter (i develop it 2 yeas ago) on the transparency document , photoshop gives a messagebox said it canot invoke the fiter because the selection area is totally empty.
if the area has both transparency and opaque pixels, then the plugin cause write access faled and made the photoshop crash ,  why does this happen, how to avoid it.
Thanks a lot.!

Answer 1

First off, don't confuse "alpha" with "transparency" they are different.
If I create a RGB document with one layer and call the Dissolve example I get the following bits of information in the FilterRecord:
planes = 4
imageMode = 3 // plugInModeRGBColor
filterCase = 4 // filterCaseEditableTransparencyNoSelection
inLayerPlanes = 3
inTransparencyMask = 1
plane 0 = red, 1 = green, 2 = blue, 3 = transparency
See DoFilter, the loop on the planes, and DissolveRectangle, the changing of the data on each plane, in the Dissolve.cpp example file.
Now flatten the document and create an alpha channel and select all channels via the channels panel. Selecing just the alpha channel or any one channel will have different results.
planes = 4
imageMode = 3 // plugInModeRGBColor
filterCase = 2 // filterCaseFlatImageWithSelection
inLayerPlanes = 0
inTransparencyMask = 0
inNonLayerPlanes = 4
There is some documentation about FilterCaseInfo in the Plug-in Resource Guide.pdf found in the documentation folder.