How to convert a 24bit rgb to 8 bit rgb

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Hello Friends,

can you help me to convert a 24bit RGB image to 8 bit RGB image...

and 16bit grey scale to 24 bit RGB..

Thanks,
PS: i am new to image programming and learning new aspects

What I have tried:

i have tried to shift bits as below ...
r = r >> 1;
g = g >> 2;
b = b >> 1;

Posted 8-Feb-16 0:00am

tasumisra

Updated 22-Jun-19 15:36pm

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2 solutions

Solution 2

Code for VB.NET

Private Function ColorRGB8bits(ByVal RGBColor As Color) As Byte
        'Converts RGB (Hex 24 bits) to 8-bit color 
        '(3 bits for red, 3 bits for green and 2 bits for blue) 
        'To convert 8bit [0 - 255] value into 3bit [0, 7]
        'the 0 Is Not a problem, but remember 255 should be converted To 7 
        'so the formula should be Red3 = Red8 * 7 / 255.
        'To convert 24bit color into 8bit (RRRGGGBB):
        '8bit Color = (Red * 7 / 255) << 5 + (Green * 7 / 255) << 2 + (Blue * 3 / 255)

        Dim r, g, b As Byte
        Dim RGB8BIT As Byte
        Dim RGB24BIT As Long

        RGB24BIT = RGBColor.ToArgb And &HFFFFFF 'Mask with &H00FFFFFF to remove the Alpha

        b = RGB24BIT And &HFF
        g = (RGB24BIT >> 8) And &HFF
        r = (RGB24BIT >> 16) And &HFF

        RGB8BIT = ((r * 7 / 255) << 5) + ((g * 7 / 255) << 2) + (b * 3 / 255)

        Return RGB8BIT
    End Function

Posted 22-Jun-19 15:37pm

jartinez

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CPallini · Accepted Answer · 2016-02-08T02:01:00

Solution 1

As noted for instance here "colors - How to convert 24 bit RGB to 8 bit RGB - Stack Overflow"[^] the 8 bit per pixel format is usually used as palette index.
However, suppose you have the following formats:

C

bit number 23 22 21 20 19 18 17 16 15 14 13 12 11 10  9  8  7  6  5  4  3  2  1  0
           b7 b6 b5 b4 b3 b2 b1 b0 g7 g6 g5 g4 g3 g2 g1 g0 r7 r6 r5 r4 r3 r2 r1 r0

bit number  7  6  5  4  3  2  1  0
           B1 B0 G2 G1 G0 R2 R1 R0


Then 
     B1 = b7, B0 = b6
     G2 = g7, G1 = g6, G0 = g5
     R2 = r7, r1 = r6, r0 = r5

that is 

     b = (rgb >> 16) & 0xFF;
     g = (rgb >> 8) & 0xFF;
     r = rgb & 0xFF;

     B = b >> 6;
     G = g >> 5;
     R = r >> 5;

     RGB = (B << 6) | ( G << 3 ) | R;

Posted 8-Feb-16 2:01am

CPallini

Comments

tasumisra 8-Feb-16 10:54am

Thank you so much Pallini,i have been waiting for your answer and i know you are champion :) can you please help me with some good tutorial\links for the same...

apart from that how to convert 16bit grey scale to 24 bit RGB ... just exploring few things around

CPallini 8-Feb-16 16:32pm

See here
http://stackoverflow.com/questions/835753/convert-grayscale-value-to-rgb-representation
The only difference is your image is 16-bit greyscale, so you have to perform an additional preliminary step:
grey = grey >> 8

jartinez 22-Jun-19 21:33pm

Your solution is not correct. To convert 24 bit color to 8 bit color you must consider that 255 should be converted to 7, so the formula should be Red3 = Red8 * 7 / 255
8bit Color = (Red * 7 / 255) << 5 + (Green * 7 / 255) << 2 + (Blue * 3 / 255)

With your example color #c0c0c0 would lead to a wrong 8 bit RGB. Test it with this link:

http://neildowning.com/HEX_to_RGB_color_converter.php

CPallini 23-Jun-19 4:05am

My solution is correct, of course :-D
0xC0C0C0
b = 0xC0 = 10100000b
g = 0xC0 = 10100000b
r = 0xC0 = 10100000b

B = b >> 6 = 00000010b
G = g >> 5 = 00000101b
R = r >> 5 = 00000101b

RGB = (B << 6) | ( G << 3 ) | R = 10101101b

That is 10110110b with bit reversed.