Hi All,

Currently I have a very old application based on 8-bit color depth on Xsun on Solaris platform which displays gray images.

At present its working fine but I want to improve the image quality by going to 24-bit mode, so that I can get more shades of gray color.

I am a newbie in Xlib programming so don't know how to get gray color for my image in 24-bit color depth.

In 8-bit, I used to allocate <=256 pixels from the default colormap and then fill them with gray colors and store this pixel values in a array.

But in 24-bit, I am not sure what to do because it has 16 million colors, I can't allocate them all and can't store them in an array as it doesn't seem to be the logical solution.

I will appreciate if some one provides me the right pointers, to proceed further,as I am really stuck with this problem.


Regards,
Anurag

A 24-bit colour is made up of three 8-bit values, Red, Green & Blue. If I remember correctly they come in that order. For grey scale the Red, Green and Blue values will all be equal, hence a (128, 182, 128) triplet will be 50% grey. The (128, 182, 128) triplet will have a hex value of 808080.

In addtion to what graemef says, there are only 256 shades of gray in a 24-bit color environment. Surprisingly, this is exactly equal to the number of grey shades you could have in an 8-bit monochrome system.

In this case it could suffice to create any array with 256 entries each containing a value with equal values vor R, G and B.

jlinkels

graemef and jlinkels,

Thanks for your reply. I really appreciate your response on my issue.

But I am little confused when you say we can have only 256 shades of gray color even on 24-bit color depth. As per my understanding, I thought we will have more shades of colors in 24-bit as compared to 8-bit.

If I still get 256 gray colors in 24-bit, can I expect my image quality to improve.


Also, I am using Xlib/Xwindows calls to display my image, Can I have small program to understand it better as I am still not sure how to modify my code to make it work for 24-bit?

If required, I can provide litle more details on my implementation.

Anurag

You will only have 256 shades to choose from with the extremes being black and white. However you have the whole depth of different colours as well and certainly some colours that are not true greys may appear that way, for example (128,127,129) #807F81 is not a true grey because it will have more blue than green but it "looks" like a grey.

If you have access to kde look at the KColorChooser program to get an idea of how 24-bit colour works.

In 24 bits your color triplets for gray are 010101, 020202, 030303... fefefe, ffffff. Those are really just 256 different values. There is no way to get more gray values, unless you want to try graemef's suggestion, which is mathematically not gray although it might improve your perception.

You get more shades of gray in 24-bit color than in 8-bit color, but the same as in 8-bit monochrome. I think the older Sun's were monochrome. In monochrome you have only one color value, and that happens to be gray (or green, or amber...) So your value is 01,02,03...fe,ff. Exactly the same as gray values in 24-bits.

Old Windows(TM) systems could only display 256 colors at the same time. Even then it was possible to have all of those color be a shade of gray, just by filling the palette only with equal values for R,G and B.

jlinkels

oops, posted twice

24BPP has the same amount of gray as 8BPP with an all gray palette! you need deep color if you want more.

graemef thanks for the suggestion but I don't know where to get the KDE code.

I trying searching it on the web, but couldn't get anything relavant.

I will be happy to know any other place where I can get this code or any sample code.

It's right here.

Hope this helps.

Hi,

Thanks for all your inputs, I got some theoretical idea about gray colors in 24-bit format.

Can anybody post a small code to show it using Xlib calls?

Basically, I want to get 256 different shades of gray colors, and then create array of pixels for these colors.

I guess we can use XParseColor or similar function for doing it, but how to do it for 256 different gray shades in loop.

I know for a single rgb string we can do something like this.

if(XParseColor(display, default_cmap, "rgb:EE/EE/EE", &fontcolor)){
XAllocColor(display, default_cmap,&fontcolor);
}

and then use fontcolor.pixel value to fill my pixel array.

but how to create different rgb strings from 00/00/00 to FF/FF/FF in a loop and pass it to XParseColor function.

I hope I am able to explain my problem clearly.

Thanks,
Anurag

possibly
or maybe