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This appears to be a Photoshop defect. I am looking for a simpler workaround.
Photoshop stores 32-Bit BMPs with a white matte instead of retaining the original color information and varying the opacity. My application uses 32-Bit BMP with alpha channel transparency and the white-biased translucent pixles are obvious.
This defect is masked by an odd behavior of Photoshop; when loading a BMP with transparency, the transparent areas are filled with white. This behavior contrasts with loading the same image saved as PNG which correctly preserves transparency. If Photoshop were to correctly display a 32-Bit BMP, this defect would be easy to spot.
It isnt easy to verify this defect because not many applications handle transparency in BMPs. While loading images with a dropshadows stored as 32-Bit BMPs using PS, my application shows a white halo around the drop shadow. I cannot work around this in my application because the original color information in the translucent areas is lost. Examining the BMP’s binary data, there is a "fade to white" as opacity drops.
My current workaround is:
– merge visible areas of an image with translucent areas – duplicate this merged layer
– duplicate and merge the duplicated layer to reduce the opacity of translucent areas until they are completely
opaque (12 times).
– move layer under the original merged layer
– Ctrl-Click (Command-Click) the original merged layer
– On channel palette, "Save selection as channel" – Enable new Alpha channel
– Save as 32-Bit BMP (A8R8G8B8 or X8R8G8B8 only makes a
difference in the BMP header)
A BMP saved this way still loads with a white background in Photoshop, but the underlying color of the multiple-merged layer is stored in the alpha blended pixels instead of fading to white. My application shows that the white halo is gone and anti-aliased edges and translucent areas display correctly.
If anyone would like a tool to verify that Photoshop saved 32-Bit BMP images have white halos, I can send a Windows App that demonstrates it. I would appreciate any input on a simpler way to workaround this or maybe the Preferences setting to Disable White Mattes for A8R8G8B8 BMP images that I overlooked.
– Stan
Photoshop stores 32-Bit BMPs with a white matte instead of retaining the original color information and varying the opacity. My application uses 32-Bit BMP with alpha channel transparency and the white-biased translucent pixles are obvious.
This defect is masked by an odd behavior of Photoshop; when loading a BMP with transparency, the transparent areas are filled with white. This behavior contrasts with loading the same image saved as PNG which correctly preserves transparency. If Photoshop were to correctly display a 32-Bit BMP, this defect would be easy to spot.
It isnt easy to verify this defect because not many applications handle transparency in BMPs. While loading images with a dropshadows stored as 32-Bit BMPs using PS, my application shows a white halo around the drop shadow. I cannot work around this in my application because the original color information in the translucent areas is lost. Examining the BMP’s binary data, there is a "fade to white" as opacity drops.
My current workaround is:
– merge visible areas of an image with translucent areas – duplicate this merged layer
– duplicate and merge the duplicated layer to reduce the opacity of translucent areas until they are completely
opaque (12 times).
– move layer under the original merged layer
– Ctrl-Click (Command-Click) the original merged layer
– On channel palette, "Save selection as channel" – Enable new Alpha channel
– Save as 32-Bit BMP (A8R8G8B8 or X8R8G8B8 only makes a
difference in the BMP header)
A BMP saved this way still loads with a white background in Photoshop, but the underlying color of the multiple-merged layer is stored in the alpha blended pixels instead of fading to white. My application shows that the white halo is gone and anti-aliased edges and translucent areas display correctly.
If anyone would like a tool to verify that Photoshop saved 32-Bit BMP images have white halos, I can send a Windows App that demonstrates it. I would appreciate any input on a simpler way to workaround this or maybe the Preferences setting to Disable White Mattes for A8R8G8B8 BMP images that I overlooked.
– Stan
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