How to fix uneven/gradient lighting on a canvas with white background?

Question

I have a photo of a pencil drawing made on paper, taken with a hotshoe flash on the DSLR and two polarising filters (cross polariser technique for canvas photography).

The problem now is that when getting closer to the drawing, lighting becomes uneven since the lens casts a shadow on part of the image. This creates a gradient over the image that is approximately, but not quite, linear.

Below is an example image; the original is around 3 cm wide.

How can this be edited such that the background is evenly white?

—Edit—

Scanned results below, uncorrected and corrected. While this looks quite promising at first, it unfortunately is full resolution what you see here (600 px wide); the scanner can do 300 dpi, and the drawing is roughly one inch wide. This is a bit less compared to the 5000 px from camera.

Also, the uncorrected scan hints that graphite indeed reflects a good part of the light, as opposed to the cross-polariser technique.

Answer 1

The method I've used myself is similar to yours, but uses the Resynthesizer plug-in (for GIMP) or Content-Aware Fill (for Photoshop) to reconstruct the gradient:

1. Create a selection that completely covers the drawing. You can do this by hand, or you can use high-pass filtering to compute a selection mask like this:

   • Start by using an edge-detection filter like Difference of Gaussians on (a copy of) your scan to crudely extract the drawing:

     

   • If the result lacks contrast, use the Levels tool to darken it:

     

   • Apply some Gaussian Blur:

     

   • Use the Threshold tool to get a mask covering the entire drawing, and use Select by Color to select it:

     

2. Once you have a selection covering the drawing, make a copy of the original scan layer and use Heal Selection / Content-Aware Fill to fill it in. Ideally, your copied layer should now look like a picture of a blank sheet of paper:

   

3. Set the edited layer's mode to Divide, as in your own answer. If you wish to retain some of the paper texture and/or shading, you may wish to reduce the layer's opacity a little, and perhaps apply a small amount of blur:

   

   (The picture above has the filled-in layer Gaussian blurred by 10px, and opacity set to 95%.)

4. Optionally, after merging the layers, adjust the levels to set the black point (tip: use the logarithmic histogram view) and increase contrast:

   

^{(Ps. Click the half-size images to view them at full size.)}

The nice thing about this method is that it can work fairly well even for quite non-linear shading gradients. Of course, it's not perfect — if the paper has, say, an irregular stain whose boundary lies partly under the drawing, the content-aware fill is unlikely to reconstruct it correctly. Still, as seen above, it often gives pretty decent results.

Answer 2

Edit: I wrote a Gimp script that does the steps below, and another one for Ilmaris answer. Both scripts are available for download on GitHub. The suggested way to go is this one.

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I’m answering this right here because I have been searching for a solution for quite some time and found a simple and working one. Let’s get to the result right away:

1. Create a background layer by copying a vertical stripe of the background only and stretching it to the size of the original image.

   In Gimp: Select r a rectangle from top to bottom containing background, paste Shift+Ctrl+V to a new image, and expand Filter > Map > Tile it to image size, run a gaussian blur over it, then copy it back to the image as new layer.

2. Set the layer mode to Division.

3. Reduce layer opacity as desired, e.g. to 90 %. Keeping it on 100 % is usually a bit too bright.

What this does, in essence, is that the background layer defines the RGB value w of “white” on each pixel. The division layer mode then stretches the value on the image from [0,w] to [0,255] (see GIMP layer modes) per pixel.

(Bad) Example using curves and linear gradient

First attempts were always about duplicating the layer, changing the brightness such that the darkest part on the bottom were lit properly, and adding a layer mask with a linear gradient to lighten up the darker regions only. The stronger the gradient, the worse it looked.

(Better) Example using division

Division yields much better results; lighting is fixed evenly, and there are no burnt areas as especially in the left part of the drawing.

Answer 3

This answer describes the fastest reliable approach I know atm, based on the answer by Ilmari Kanoren. It is semi-automatic; the automatic mask does not work for hard images like the one below because there are no edges in some parts of the drawing.

This Gimp script automates steps 3 and 4 (note that Resynthesizer is required), so the workflow is just: Create selection, run script. It takes 6 seconds for the script to complete a 15 MP image on my 3-years-old laptop.

Step 1 (optional): Adjust curves for deep black

Step 2: Select the object

In Gimp, the Quick Mask Shift+Q is a fast way. Use the pencil N and paint the object white, press Shift+Q again to convert the mask to a selection.

Step 3: Recreate the background

• Duplicate the image and scale to 400×400 px
• Use Heal Selection or Content-aware fill to heal the object away (only background remains)
• Use Gaussian Blur, 40×40 px
• Scale the image back to original size and copy it back as layer to the original image

Step 4: Division mode

Set the layer mode to Division and reduce opacity a bit, if desired.

Answer 4

This is the photography forum, so I will take a photographic approach. Of course for a perfect white you probably still need some post production tweaks.

Instead of using a flash you can try using natural light. Avoid direct sunlight, don't stay too close to the window either, but like 2 meters away from the window. This is to reduce the difference in light between the closest and furthest part of the drawing. (But as this is a small image that distance may not be that important.)

You can point a tripod facing down and put your drawing on the floor.

Or if you don't have a tripod you can arrange a book cover or a cereal box to hold the image vertically on a table, and put your camera steadily on the same table. (Put a white sheet of paper behind your drawing to avoid seeing the background images or textures.)

The polarizer technique you are mentioning is in the case you have the drawing behind a glass, and when you have bright objects or reflections. But if your source light is in an oblique angle you probably don't need to use a filter.