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Old 10th June 2001, 12:14 AM   #1
Abbott Katz
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Lightbulb Bit depth change

Hi, all. Simple question: How do I chang the bit depth of a JPG in PhotoPlus from 24 to 8?

Thanks!
 
Old 10th June 2001, 02:56 AM   #2
Jim B.
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Lightbulb Re: Bit depth change

Convert the JPG file to, or save it as, a GIF file.

GIF is 8-bit (256) colors.

Note that if your original JPG file is a photo or "photo realistic", then converting it to 8-bit will seriously degrade the image.

Jim B.   :>)
 
Old 10th June 2001, 12:27 PM   #3
Abbott Katz
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Lightbulb Re: Bit depth change

Thanks much for your reply; I'll reply in turn with another obvious question: Many web photos - apparently saved to JPG format -- exhibit high resolution even as they report a relatively low byte count. I can't seem to do the same -- that is, hammer out a handsome photo while at the same time lessening the byte total. How can I do just that? Thanks again.

Abbott
 
Old 10th June 2001, 03:02 PM   #4
Martin Taylor
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Lightbulb Re: Bit depth change

Hi - you're on to one of my niggles about Ph+ ...

You can save space in a jpeg by reducing the resolution (dpi). There's no point in having a dpi setting more than about 75 for most monitors - they can't display higher resolution anyway. Mind you, I have seen 96 recommended as a guideline too, for online display.

I've just tried saving the same image as 150 dpi and then as 75 dpi. It goes down from 135K to 35K as a result, and looks just as good.

Now - the problem is that if you export a Ph+ picture as a jpeg, it always saves as 150dpi. Even though you can allegedly choose the dpi in the Image/Resize dialog, Ph+ ignores this during the export process. The result? Bloated jpegs.

Try loading the jpegs into (say) PhotoImpact, change the resolution to 75dpi, then save again.

Any use?
 
Old 10th June 2001, 03:46 PM   #5
Jim B.
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Lightbulb Re: Bit depth change

To reduce the <I>file</I> size of a JPG file, you <I>increase</I> the amount of compression at the time when you <I>(re)save</I> the file. A compression of 60 - 80 quality (or 20 - 40 percent) is often a reasonable compromise between smaller file size and less quality loss. 100 quality, or 0 percent is minimal compression and maximum quality

Note that JPG compression is a <I>lossy</I> compression. That is, the more you increase the compression, the more fine detail and smooth color changes that you will permanently lose. NEVER work on your original JPG file, always edit a COPY, and save it with a name that is different than the original.

The Dots Per Inch (DPI) that (some) image editor programs report has absolutly <I>no</I> effect on how the image is displayed on your monitor. If you are not printing the image, don't even think about DPI, it will just confuse you.

Your computer is set to display the screen at a certain <I>resolution</I>, and that resolution does not depend on any physical dimension like inches or centimeters.

Here is how to see what resolution your computer is set to display. In a Windows 98 system, RIGHT-click on any blank area of the desktop, then click on "Properties", then the "Settings" tab, then look at the slider in the "Screen area" part of the window. Mine is set at 1024 x 768.

Also see the "Colors" box? It should be at 24-bit (16.7 million) colors or more when you are viewing photos, because photos should be (are usually) in JPG format, which is 24-bit color. Using less than 24-bit color will cause areas of similar color in the photo to appear "banded" on your monitor.

What a "Screen area" setting of 1024 x 768 means is a 640 x 480 photo will occupy less than the full screen. A 1024 x 768 photo will exactly fill the screen. A 1600 x 1200 photo will be larger than the screen, and you will have to use the scrollbars to see the entire photo.

But you also need to know that not all video cards can support higher (24- or 32-bit) color depth at the higher (1024 x 768 or larger) resolutions. So don't be surprised if you can't get some resolutions at certain color depths (or conversely, some color depths at certain resolutions) on your computer.

Also, the size of the monitor makes no difference as to the resolution of the video card. If you set the "Screen area" to 640 x 480, then display a 640 x 480 photo, it will completely fill your screen, regardless of the size of the monitor you are using. If you set the "Screen area" to 1600 x 1200, then display a 1600 x 1200 photo, it also will completely fill your screen.

If you set the resolution to 1600 x 1200 and you are using a 15-inch monitor, everything will be veerrryyy tiny on your screen. If you set the resolution to 640 x 480 and you are using a 19-inch monitor, everything will be veerrryyy HUGE on your screen.

Jim B. &nbsp; :>)
 
Old 11th June 2001, 08:41 PM   #6
Abbott Katz
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Lightbulb Re: Bit depth change

thanks very much for your analysis!

Abbott
 
Old 11th June 2001, 08:41 PM   #7
Abbott Katz
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and much thanks to you for your multiple replies.

Abbott
 
Old 13th June 2001, 03:25 PM   #8
Martin Taylor
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Jim - you're right ... I was confused by the apparent reduction in file size when changing resolution, but I'd missed that the compression percentage had changed too (unseen by me).
 
Old 14th June 2001, 10:35 AM   #9
Dale Cook
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Lightbulb Re: Bit depth change

Changing the actual image size, in pixels, helps the file size considerably. For Web sites if an image is intended for use at approximately 2"x2" at average display resolution, you can resize the image accordingly, making it perhaps 200x200 pixels. Sometimes a large image is squashed into a small space by the Web design program, which leaves the file at its larger original size.

Compression, as Martin mentioned, is hugely important in the case of JPG. For the sake of a minor change in quality the file size can be reduced considerably. Yet more size can be cut if the quality reduces heavily, but a minor change in quality is normally enough for you to note a big change in file size. Image DPI is a function of printing, as most displays are fixed at 96dpi, leaving just the number of actual pixels as the influence on file size.

Hope that helps tie the advice together. Dale.
 
Old 17th June 2001, 02:14 PM   #10
Abbott Katz
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Again, thanks to all for a most informative interchange. I still cannot quite realize the same byte efficiencies that other on-line photos appear to carry off, but I'll continue to try. Thanks again.

Abbott
 
Old 17th June 2001, 06:36 PM   #11
Kevin
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Lightbulb Re: Bit depth change

Hi all,
This is my first time on this site. The subject of resolution does confuse me somewhat. I like to make sure that if I am editing my photos i.e. cropping, it helps if I resize the image to 500ppi, 24bit/32bit so that the finished work maintaines a high degree of resolution. The drawback is that file size can be quite large -some in excess of 100mb at times.
Am I doing something wrong here, I have only used PP7 for two weeks!
 
Old 18th June 2001, 08:01 AM   #12
Dale Cook
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Lightbulb Re: Bit depth change

Hi everyone.

Resolution does seem to be confusing in the outset, but it is really simple, honest. Focus on the measurement term for resolution, "dots per inch". "Per inch" means that it is a function of distance or space, not just dots, which are what image-editors are concerned with. For simplicity, consider DPI a print term, not a photo-editing term.

A photo on screen in a photo-editing package only has dots, really, not dots per inch (simplification). All you need to do is force the image to be comprised of a suitable number of dots relevant to the desired output medium and size. Thousands of dots are only needed for printing large high quality pictures. You can use a couple of hundred dots or less for an image which is to be used for the Web.

A 100MB image at 32-bit depth is going to be roughly comprised of 26 million pixels (e.g. 5120x5120 pixels). Given that a typical photo from an expensive digicam is about 3 or 4 million pixels, 26 million pixels in a single image seems overkill. But there are some times for some people when a massive number of dots, i.e. high quality images, are required. When the need arises in my job I produce images suitable for professional printing at 300 DPI, so when I export product boxshots for magazines for example I generally make the image roughly 1000x1000 pixels first. I can also conceivably be making buttons for a tutorial that are just 18 pixels wide. File size isn't an issue for those :-).

Most people print at 300 DPI at home too, so images don't often <i>need</i> to extend into thousands of pixels in each direction.

That's a suitable summary of when DPI is important - at print time, but consider how many actual pixels will be ultimately needed for output while you're at the image-editing stage.


Dale
 
Old 18th June 2001, 07:21 PM   #13
Kevin
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Lightbulb Re: Bit depth change

Dale,

Does the same apply to printing A4 size quality enlargements from Kodak Picture CD, which is what I am using most of the time due to my 35ml equipment. File size is around the 1mb size per photo.

I do experience a degree of pixelation if I serverely crop an image, which I assume would be present on the end print, but if I alter the image resolution from the default 150ppi to 300, 500 or even 1000ppi the pixelation problem disappears resulting in a good quality print to A4 size only limited by the film grain size.

Do you think that this type of editing is still regarded as overkill?

Kevin.
 
Old 18th June 2001, 10:45 PM   #14
RL
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Lightbulb 256 colors

The messages that are here already aren't helping me. My question is, how do I convert my work to 256 colors, and then back to the original bit depth again?
 
Old 19th June 2001, 02:02 AM   #15
Jim B.
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Convert to 256 color then back to the original bit depth? Hmmm.

What are you converting? An image file? A graphic file? Your screen display?

If you convert an image file from <I>more</I> than 256 colors down to 8-bit (256) color, you will lose all the colors except for 256. Then if you increase the bit depth, you will still have just those 256 colors.

If you convert a graphic file from <I>less</I> than 256 colors up to 8-bit (256) color, you will still have only those few colors that were in the original file. Then if you reduce the bit depth, you are back to where you started.

If you change your screen's color display (bit depth), that will affect only how the file is displayed on your monitor, and not the file itself.

To convert an image (typiclly a JPG) file to lower bit depth, use an image editor like PhotoPlus to reduce the colors, or use the Save As option to save the file as a GIF.

To convert a graphic (typically a GIF) file to higher bit depth, use an image editor like PhotoPlus to increase the colors, or use the Save As option to save the file as a JPG.

To change the bit depth of your computer monitor, RIGHT-click on a blank portion of the desktop, then click on Properties, then the Settings tab, then select your desired bit depth from the Colors dropdown box.

Just out of curiosity, what do you expect to gain, or be able to do, by converting to 256 colors then back?

Jim B. &nbsp; :>)
 
Old 19th June 2001, 07:00 AM   #16
Dale Cook
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Lightbulb Re:256 colors

Many PhotoPlus features would need to be disabled if we offered a 256-colour <i>mode </i> to work with and export from. Instead, all PhotoPlus tools work all the time because PhotoPlus always uses 24-bit colour depth editing.

Knocking an image down to 256 colours is something that's achieved during Export. You'd need to create a 256-colour (8-bit) <i>and </i> a 24-bit version of your image if you wanted to be able to work with both colour depths.

Regards, Dale.
 
Old 19th June 2001, 07:14 AM   #17
Dale Cook
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No, a 1MB image at A4 size is far from overkill - no wonder you increase the number of pixels when working at this scale. I was referring to typical useage - it's difficult to include all types of work in one statement. I also didn't touch on the technicalities behind increasing an image's size for this kind of work; the addition of new pixels by a resize routine is less "accurate" than having a large number of pixels to start with, which is why we and others offer different mathematical methods of generating these new pixels.

Dale
 
Old 19th June 2001, 06:31 PM   #18
Kevin
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Lightbulb Re: Bit depth change

So, what is the best way of generating new pixels from a small source file?

Kevin
 
Old 20th June 2001, 05:23 AM   #19
Dale Cook
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Lightbulb Re: Bit depth change

Slide the Quality slider to the right in the Resize dialog, so that PhotoPlus uses Bicubic Interpolation. This method of interpolating can also be used for the Mesh Warp tool, Deform Tool etc.

This mathematical algorithm is slower than Bilinear or Nearest Pixel interpolation, but produces better results. It's useful but not as accurate as having enough pixels in the first place. But that's why it's offered as a tool, because we often need a bigger image than we start with!

Dale.
 
Old 21st June 2001, 07:00 PM   #20
Kevin
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Lightbulb Re: Bit depth change

I tried moving the slider to the right using Bicubic Interpolation with a 1mb image and 150ppi. When I zoom in 5 mouse clicks I see that the pixels are just as obvious as if I had'nt selected anything, but if I resize the image up to 300ppi the pixalation dissapears until you reach 7 clicks i.e. vastly improved image particularly when cropping.

Am I missing something with this slider because when I undo and redo after selecting it, there is no noticable difference.

Kevin.
 
 

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