Strange color artifacts with Overlay()

[2E7AH]

Consider this code, which doesn't make sense but I simplified larger code as I could:

file = AviSource("clip.avi", audio=false)
drop = ImageSource("empty.png", pixel_type="rgb32")

y = Height(file)

Overlay(BlankClip(file, Height = 7*y, Width = Width(file), color=$ffffff), drop)

Layer(Trim(file,434,0),"add",256,0,0)
Overlay(drop, 0, 0, mask=ShowAlpha(Mask(drop, drop.ShowAlpha(pixel_type="rgb32"))))

Layer(Trim(file,434,0),"add",256,0,y)
Overlay(drop, 0, 0, mask=ShowAlpha(Mask(drop, drop.ShowAlpha(pixel_type="rgb32"))))

Layer(Trim(file,434,0),"add",256,0,2*y)
Overlay(drop, 0, 0, mask=ShowAlpha(Mask(drop, drop.ShowAlpha(pixel_type="rgb32"))))

Layer(Trim(file,434,0),"add",256,0,3*y)
Overlay(drop, 0, 0, mask=ShowAlpha(Mask(drop, drop.ShowAlpha(pixel_type="rgb32"))))

Layer(Trim(file,434,0),"add",256,0,4*y)
Overlay(drop, 0, 0, mask=ShowAlpha(Mask(drop, drop.ShowAlpha(pixel_type="rgb32"))))

Layer(Trim(file,434,0),"add",256,0,5*y)
Overlay(drop, 0, 0, mask=ShowAlpha(Mask(drop, drop.ShowAlpha(pixel_type="rgb32"))))

Layer(Trim(file,434,0),"add",256,0,6*y)
Overlay(drop, 0, 0, mask=ShowAlpha(Mask(drop, drop.ShowAlpha(pixel_type="rgb32"))))

where empty.png is png with transparency

Result:
http://img138.imageshack.us/img138/6296/clipboard01n.th.png (http://img138.imageshack.us/img138/6296/clipboard01n.png)

Now, if we use this only once:

Layer(Trim(file,434,0),"add",256,0,0)
Overlay(drop, 6, 3, mask=ShowAlpha(Mask(drop, drop.ShowAlpha(pixel_type="rgb32"))))

it's almost like nothing happened, but as we repeat that function, the ugly artifact grows on and on.

Can someone tell me why is this happening?

Thanks

[Dark Shikari]

Looks like artifacts from repeated colorspace conversion maybe?

[2E7AH]

Thanks for the fast response :)

I don't know if I understand you right: there is no conversion in above example code, and it happens with any clip I try
Here is another example (I needed to use ConvertToRGB here for the clip source as it is in YUY2)

http://img63.imageshack.us/img63/9947/crashf.th.png (http://img63.imageshack.us/img63/9947/crashf.png)

How can I avoid this?

[wonkey_monkey]

Overlay converts to YUV internally, so maybe it's compound rounding errors?

David

[2E7AH]

So it seems that using Overlay() more than once in the same scene is not good idea
I managed to use Layer() instead

Thanks for explanations