Converting BMP to Grayscale–Histogram Problems

MP
Posted By
Megan_P
Nov 20, 2006
Views
607
Replies
12
Status
Closed
Hi,

I’m having a rather bizarre issue! What I’m going to describe happens both on my Mac at home (iBook G4) and on the PCs at work (they’re Dells, but I’m not sure what model).

I have a large group of bitmap tiff images that were scanned from microfilm. I want to adjust the images so that the parts that look washed out or very light will be darker/more visible. I’ve converted all the files to grayscale, but still am not able to adjust them (using levels, curves, filters, or by painting on layer masks) because CS2 doesn’t seem to recognize that they have histograms. When I bring up the Levels box, for example, no data shows; if I move the sliders anyway the preview of the image shows adjustment, but then when the task is completed the image goes back to looking washed-out. The really odd thing is that when I convert the images I get a dialog box that says "building histograms," and the histogram *palette* thereafter shows data. But, if I re-load the histogram in the palette and discard the cached info, it comes up blank. Anybody have any ideas on how to fix this problem–or work around it so that I can adjust my images?

Thanks a lot.
Megan
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bmp conversion grayscale histogram problems

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CB
charles badland
Nov 21, 2006
Megan,

Your image contains only black or white pixels. The histogram in the Levels dialog is not blank, there is one single line on the far right and a single line on the far left (showing the black and white pixels.) The false preview is probably caused by not viewing at 100%.

How can you fix it? Maybe try adding a slight Gaussian Blur to introduce some gray tones, then use Levels to lighten or darken. View at 100% to see what is really hapening to your image.
JS
John_Slate
Nov 21, 2006
Why should she blur her images if they only need 2 values?

For that she should use bitmap mode, provided she has enough resolution (might be tough from microfilm!)
CB
charles badland
Nov 21, 2006
She came from Bitmap Mode. I’ve run into the same issues with clients who have one bit scanned image where the lines/characters are too "thin" and the overall document looked "washed-out". One way to give a little more weight to the lines is how I described. Destructive? A little, but they were happy with the results.
It is also common for people not used to working on a 1 bit file (converted to grayscale) to be confused why the levels and curves and preview don’t work as expected.
JS
John_Slate
Nov 21, 2006
What about selecting the weak elements, expanding the selections, and filling with black?

Then reverting back to 1 bit.
CB
charles badland
Nov 21, 2006
That too. But it would seem like more work and have fewer options. And I don’t see where Megan is necessarily trying to revert to 1 bit.
MP
Megan_P
Nov 28, 2006
Thanks, Charles and John. The gaussian blur works perfectly for my problem. One more quick question, though (or, at least, I think it’s quick): I’d also like to add a layer mask and darken specific parts of the images even more; what I’ve done in the past is to fill the mask with black and then paint on the layer with white to bring out the black from the mask (I hope that makes sense.) This tactic isn’t working with my images, even after the blur is applied.

Also, I won’t be reverting back to 1-bit; the images will stay greyscale.

Your help is *much* appreciated! I was tearing my hair out over that first bit. Megan
CB
charles badland
Nov 28, 2006
I guess I don’t understand how you are using the mask… with an Adjustment layer? Like Levels or Curves?
MP
Megan_P
Nov 29, 2006
Well, I’m using the mask on a layer I’m *calling* an adjustment layer, but it was created by selecting parts of the background layer and creating a new layer "by copy." So, on this new layer, after I blur and adjust curves, I’m adding a layer mask. Then I’m filling the layer mask with 100% black, and then trying to paint on the mask with white, which has in the past worked to darken those parts of the image that are too light by an almost silkscreen-like effect. Does that make any more sense? (As you can tell, I’m a bit of a newbie, at least with CS2!) 🙂

The images, to give you a bit more detail, are of old ballad sheets that contain both text and pictures; the text does not need to be adjusted at all but the pictures (which were originally b/w woodcuts) are hard to see. Within these woodcut pictures, certain areas are lighter/more "washed out" than others, and I want to bring everything up to a reasonably consistent level. I also tried the select > expand method, but even expanding by a few pixels made the lines from the original woodcuts look strange.

Thanks again.
CB
charles badland
Nov 29, 2006
It’s hard to say without seeing what you are doing.

You say you have put part of the background image on its own layer, blurred it a bit, then darkened with Curves. Then hid that layer with a mask. But when you paint white on the mask, it is not revealing the layer you have darkened?

Did you see the effect of Curves darkening your image before creating a Hide All layer mask? Are you viewing at 100%?
B
Buko
Nov 29, 2006
It sounds like they should have been scanned as grayscale instead of bitmap.

time to rescan.
MP
Megan_P
Nov 30, 2006
Actually, I would *love* it if they’d been scanned as full color, at something higher than 400 dpi, but these microfilms were made years ago, from archives in the collection of a small library that’s very reluctant to even have people look at the originals, much less re-photograph them. So, at least for the forseeable future, this is all we’ve got. 🙁

Charles: hmmm….this is probably a dumb question, but what’s a hide all layer mask?

Otherwise, yes: I did see the effect of the levels (which I’m more comfortable with than curves, but I could try that too), though the preview image was significantly darker than the real image after levels had been applied. But, still a perceptible difference for sure.
CB
charles badland
Nov 30, 2006
but what’s a hide all layer mask?

Just what you are describing: a mask filled with black. 🙂

Don’t know why painting white on the mask is not revealing the (darkened) layer. Can you see white on the mask in the layer palette as you paint? Can you see the mask tones if you Option+click on the mask in the layer palette?

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