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16 bit BMP file with indexed color?

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Gerg Posretan
16 bit BMP file with indexed color?
on Feb 2, 2015 at 12:58:34 pm

I need to create a 16 bit BMP image for a multimedia device.
By this I mean a 16 bit file NOT 16 bit per channel.
The file has a diagonal colored gradient.
When the image is created as an RGB 8 bits per channel and saved as a 16 bit bmp file the gradient is horribly banded.
If I make the image "indexed color" I can only save it as an 8 bit file. The 8 bit file with 255 colors looks fine BUT the device will not accept it as it is not 16 bit bmp.
Is it possible to create a 16 bit bmp file with indexed color?
..... and of course HOW?
I am using CS4.

Gerg.


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Lasse Perälä
Re: 16 bit BMP file with indexed color?
on Feb 2, 2015 at 10:12:00 pm

Strange, I couldn't figure it out. Try a different program, GIMP for example.


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Jonathan Ziegler
Re: 16 bit BMP file with indexed color?
on Feb 3, 2015 at 8:58:10 pm

Is this an older device using a 565 type color (in other words RGB is 565 vs 888)? There are others, too (1555 if you have transparency, etc).

First, don't bother with indexed color. This won't do what you want it to do in this case. You'll have to start with a 24-bit RGB image and, when you save, you will convert to 16-bit. It's really pretty easy.

For this, I'm using Photoshop CC 2014. I do not know if this will work in CS4, but since BMPs have been supported for a very long time (in computer terms), it should still work.

So, create your image. You can check this site for SIMULATING the image (before you save so you don't have to keep building/saving/reqbuilding to get the image just so).

http://www.edais.co.uk/blog/?p=92

Start with a regular 24-bit RGB image - the tutorial above will help a bit in narrowing color choices down, but remember your image is still 24 bit until you save as a BMP file:

When you are ready to save, go to File >> Save As, Pick BMP from the list. Choose 16bit from your bit depth. Depending on the color palette you need, you can hit "Advanced Modes" to pick - check these out anyhow as it may help your colors (you can always just take defaults and go back to Basic Modes). You have 6 different options to choose from, though they may not all be selectable. Default is X1R5G5B5 which allows a bit for transparency (I think). If your device doesn't support transparency, you may need to choose R5G6B5. Check with the device manufacturer if possible for further details on which color choices to use or which palette to pick.

I'm not sure what "Flip Row Order" does, but I'm assuming it changes the color order for the device in the file. Check with your device's manufacturer, too as they may have further instructions.

I'm curious to see how this turns out. Hope it helps. :)

Save early. Save often.

Jonathan Ziegler

http://www.electrictiger.com
520-360-8293


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Gerg Posretan
Re: 16 bit BMP file with indexed color?
on Feb 4, 2015 at 12:54:35 pm

Thanks for you help.

The results of my experiments are attached:

Top left original 8 bit per channel project file.
Top right default saving as a 16 bit file (1555)
Row 2 left red 255 black gradient saved as 565
Row 2 right green 255 black gradient saved as 565
------ you can see the different one bit makes with the green
Row 3 left blue 255 black gradient saved 565
Row 3 right green 255 black gradient saved 1555.

It is interesting (to my eyes) the banding seems less pronounced with the blue gradient. If I remember blue is not the most sensitive colour for human eyes.

The greyed out options become active if you add an alpha channel.
Quite useless actually as the RGB is reduced to 444.

Obviously what I need is a piece of software that would allow 646, which would probably correct fro the original colours orange and blue




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Herbert van der wegen
Re: 16 bit BMP file with indexed color?
on Feb 4, 2015 at 7:00:11 pm

I did some testing myself. Photoshop's bmp export lacks any dithering option, which explains your results. In theory we should be able to reduce the number of colours first, and apply a dithering method to visually reduce (most) of the banding.

My workflow:
1) create the gradient (black to red/green/blue)
2) reduce the number of colours to 32, and apply a dither method to remove the banding
3) save as a 16bit bmp.

This works really well in Photoline: the gradients looks smooth, even for 1024x1024 dimensioned images.

However, when I tried this workflow in Photoshop, I hit a snag. In Photoshop, in order to reduce the number of colours to 32, we first must switch to index mode, and set a custom number of colours to 32, with either noise or pattern dithering. So far, so good.

Switching back to RGB/8 is required to save the BMP as a 16bit one. And something goes wrong when saving the file as a bmp: an ugly bump is introduced in the gradient by Photoshop while saving.

(These additional steps are not required in Photoline, because there is no need to switch image modes: a simple "Reduce Colours" option is available in the color menu.)

Initially I believed this to be a problem introduced by the mode switch in Photoshop. I therefore installed Quantizer from XiMagic (http://www.ximagic.com/q_index.html) to circumvent that step, and reduced the colours to a dithered 32.

Then I attempted another save, and unfortunately Photoshop's bmp export destroyed the gradient again.

Therefore, it seems that Photoshop is incapable to save a proper 16bit red/green/blue bmp gradient. Which is strange, since the identical steps in Photoline produce a very nice usable result.

PS: when I import Photoline's r/g/b gradient (32 colours dithered) bmps into Photoshop, and then save out a 16bit bmp once more, there are no issues with the gradient.

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Jonathan Ziegler
Re: 16 bit BMP file with indexed color?
on Feb 4, 2015 at 7:44:47 pm

Looks like you may need to get a custom setup from your device manufacturer. What is the make and model? Do they have any contact info?

Save early. Save often.

Jonathan Ziegler

http://www.electrictiger.com
520-360-8293


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