Filter plugin - Pixels become grey when alpha set below 50 (ish)?

Question

Hi,
My filter plugin modifies the alpha channel values of various pixels and I noticed that when an 8 bpc pixel's alpha is set below 50 (or so) that the pixel ends up turning grey, as if the R,G,B values were all set to the same value. Is this Photoshop's behaviour when painting/compositing pixels that are very transparent or am I confused? :-)
Thanks,
Neil

Answer 1

Thanks for your replies.
My prefs are set to show the transparency checkerboard, and my code is directly modifying the RGBA values of 8 bpc pixels. The '50' I referred to above was the actual alpha channel uint8 value, not a percentage.
The following is the code I'm using to test with. I put it in the DoFilter() function, at line 401, in the Dissolve sample project and disabled the for loop that iterates over the planes. I'm using the CS2 SDK and am testing with PS CS3.
I'm testing with a PSD with 2 layers in RGB 8 bpc mode. The background layer is entirely black and I've tried filling the foreground layer with various solid colors to test with. I'm applying the filter to this foreground layer.
1. With a color of 255,0,0 after applying the filter the new color (in the Info palette, or with the eye dropper) is 40,0,0.
2. With a color of 0,200,0 the filter changes it to 0,0,0.
3. With a color of 200,255,0 the filter changes it to 0,40,0.
4. With a color of 180,215,87 the filter changes it to 215,215,215. This is what made me think it was turning to gray.
-------- Begin --------
// Get RGBA interleaved pixel channels
gFilterRecord->outLoPlane = gFilterRecord->inLoPlane = 0;
gFilterRecord->outHiPlane = gFilterRecord->inHiPlane = 3;
// update the gFilterRecord with our latest request
*gResult = gFilterRecord->advanceState();
if (*gResult != noErr) return;
uint8 *pixel = (uint8 *)gFilterRecord->outData;
const int32 dataRowBytes = gFilterRecord->outRowBytes;
VRect tileRect = GetOutRect();
const int32 tileHeight = tileRect.bottom - tileRect.top;
const int32 tileWidth = tileRect.right - tileRect.left;
const int32 tileByteWidth = tileWidth * 4;
for (int32 pixelY = 0; pixelY < tileHeight; ++pixelY)
for (int32 pixelX = 0; pixelX < tileWidth; ++pixelX)
pixel += 3; // Skip RGB
*pixel++ = 40;
pixel += (dataRowBytes - tileByteWidth);
-------- End --------
Thanks,
Neil