how did multiply gray on rgb color ?

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Roman

Joined: 21 Apr 2012
Posts: 1878

Roman 12 Feb 2025, 14:32

I have rgb color one byte to channel.
And gray image.

gray DB 255,255,255
rgb DB 1,120,200

gray*rgb result rgb = 1,120,200

gray DB 128,128,128
rgb DB 1,120,200

gray*rgb result rgb = 0,60,100

12 Feb 2025, 14:32

macgub

Joined: 11 Jan 2006
Posts: 353
Location: Poland

macgub 12 Feb 2025, 14:46

Perhaps something like:
gray = A_r, A_g, A_b
rgb = B_r, B_g, B_b
result = (A_r*B_r) shr 8, (A_g*B_g) shr 8, (A_b*B_b) shr 8

12 Feb 2025, 14:46

Roman

Joined: 21 Apr 2012
Posts: 1878

Roman 12 Feb 2025, 15:12

Thanks.

12 Feb 2025, 15:12

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20517
Location: In your JS exploiting you and your system

revolution 12 Feb 2025, 16:27

Using "shr 8" gives inaccurate results.

Use "/ 255" instead to retain the full resolution.

Compare:

Code:

(255 * 255) shr 8 == 254 <--- off-by-one
(255 * 255) / 255 == 255 <--- correct value

12 Feb 2025, 16:27

macgub

Joined: 11 Jan 2006
Posts: 353
Location: Poland

macgub 13 Feb 2025, 11:42

Thanks for correction.
It can be importand, but for me kind of nuance. For me shr is more convenient to use. But all depends upon concrete implementation.

13 Feb 2025, 11:42

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