Well, pretty much what the topic says: A very rudimentary guide to levelling manga scans. Note that this was written using Photoshop 7 to do the image editing. However, any later installment of the software should feature the same tools discussed in this tutorial. Click on any of the images for a larger resolution screenshot.
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Step 1: Preparing
Open a page of your choice in Photoshop.
Create a new layer and use paint bucket to make it white.
Change the opacity of this white layer to about 40%. Now you notice that the blacks are not solid, for example all over the girl's hair in panel 1.
Step 2: Blacks
Switch back to the background layer and open up the levelling prompt (ctrl+L). Then do black levelling until you see mostly solid blacks (move leftmost slider to the right). Do not overdo it, otherwise you will end up with an image like this:
Notice how the lines get jaggy and the tongue (panel 1) and hat (panel 2) become clearly overlevelled.
Step 3: Whites
Now let's take a look at the white levels. Disable the white layer you used for black levelling and zoom in on a spot. See all that dirt/texture of the paper the image was printed on?
Open up the levelling prompt and do white levelling (moving the rightmost slider to the left) until almost all the particles are removed. Again, be careful not to level too much as the grey tones and some details will be gone if you overdo it, as evidently shown here:
Step 4: Greys
Now let's take a closer look on panel 2. Even if you were particularly careful with the black levelling, your grey tones will probably end up too dark. However, not dark enough to be unable to fix it.
Open up the levelling prompt once more and, this time, move the middle slider. This is the one determining the grey levels. Move left until you're satisfied. How much you have to move it until it looks proper depends hugely on your initial scan, but usually 1,3 is a safe value to work with. No guarantees though. If done improperly, your image will look like this:
Steps 2-4 should be done in one go, so as to prevent loss of artwork. For the purpose of this tutorial, I split it up into three parts. When editing yourself, you should not use the levelling prompt more than once on the same image.
Step 5: Fine Tuning
After the initial work is done, it's time to do the fine tuning. Remember how I always recommended to do the levelling until it approximately looked good? That was to ensure we're not destroying any artwork in the process. Let's start with the blacks.
Zoom in on a spot with homogeneous areas of black. Then, enable the white layer again. You'll see that there are actually some specks that haven't turned completely black when we levelled the scan (check mouth, eyes and strand of hair). In order to fix that, we select the Burn Tool, pick a reasonably large brush size, set the range to Shadows and the Exposure to some light value, maybe 9%.
Now, carefully begin to brush over those areas and notice how the specks disappear. Ideally, you would want this result:
Eyes, hair and mouth have been turned completely black. However, due to the nature of this tool, changing all "shadows" (meaning every pixel the programme categorizes as "dark") into a darker shade, it is easy to destroy more than you're actually trying to fix. Here's some proof of that:
The tongue has been completely blackened by mistake, so be extremely cautious as to what areas you stroke over.
Moving on, disable the white layer and zoom in on another spot. Preferably one that shows lots of dust on a white background:
Choose the Dodge Tool from the menu, pick a nice and large brush, set the Range to Highlights and the Exposure to about 9%.
Now, much like we did when using the Burn Tool, carefully stroke over the areas you want to whiten. The tool works the same way as the Burn Tool, only in opposite direction. It will turn any spots the programme categorizes as a "highlight" (very light shades of grey) into white pixels. Therefore, again be careful what you stroke over, as you wouldn't want a broken hat like this:
And that's basically it. If you followed all orders given in this tutorial carefully, you'll end up with an adequately levelled image such as this:
Some groups will require you to leave the original artwork untouched. If that's the case, simply duplicate the background layer and do all the work there. That way, the person assigned to QC can always disable your levelling layer to see if you messed up by comparing it to the original scan.
Instead of zooming in on the image, I find it easier to use the Wand tool to select areas that aren't black, then zoom in and go over those with the burn tool. Same with the whites. That was back when I actually cared about a few stray pixels, which was long long ago.