How to scan large watercolour paintings and edit them on GIMP – A step by step tutorial

Large watercolour paintings can be quite tricky to photograph sometimes. The camera has to be of a very good quality, with very good lens that doesn’t distort the edges of the image, and good sensors that can catch accurately the colours of the painting, mounted on a tripod with a good light installation in order to avoid shadows or glares on our photographed image.

But even if all these exist, ( and lets say that someone invested the money to have all these equipment), cameras can never catch the texture of the paper no matter how well the painting is photographed and what the lighting conditions are.

So I ended up to the conclusion that the best way to have a perfect image of larger paintings ( from A3 size to whatever size you want) is to scan them.
The advantages are:

  • You don’t have any lens distortion on your picture.
  • The colours are better and more accurately detected.
  • In HR images is visible the texture of the paper

But how is it possible to scan large paintings, on a A4 scanner?

Below I’ll make a step by step tutorial on how to do this together with full step by step ( with photographs and screenshots ) instructions on how to scan and assemble your scanned material in order to have a perfect and high definition and resolution image, chromatically accurate, that shows also the texture of the paper, ready to use for any purpose you might need it. ( either to show your work on your web site or sent high resolution images for joining competitions and such things).

What we are going to need.

For the purposes of this tutorial I’m using an Epson Perfection V33 A4 Scanner ( an average cheap commercial scanner) and

GIMP image editor version 2.8 ( free to download and use manipulation program).

Other things that you are going to need is an A4 carton, or an A4 Sketchbook, something at A4 size in order to put it over the part of your painting you scan each time. I find very convenient to use one of my A4 hardbound sketchbooks because they are heavy enough to keep the paper in place.

Get also a pencil. (any kind of pencil).

Make sure that the glass of the scanner is clean ( and check it periodically while you are scanning ).

Scanning

Let’s start: Connect the scanner on the pc and place it on a large steady and flat surface. If your desk is not large enough place the scanner on the floor. You’ll need enough space in order to be able to move and place your large painting freely on the surface of the scanner.

Then open the lid of the scanner completely flat. On some scanners this can be done by pulling the lid up and then open it completely flat, and in some other by removing completely the lid.

Then start the scanner’s settings program on the pc and lets proceed with the scanning.

The main idea is the following.
Large paintings have (inevitably )to be scanned in parts. Then these scanned parts of the painting, have to be assembled in one and single image file.
The image file must not be that large in size in order to be able to be edited later without exceeding the processing power of the pc and end up with a final file that will be huge and so impossible to manipulate/edit easily.

So we have to set up the scanner to the best ( optimal ) resolution in order to do so.
From my experience the very high resolution for a 56×76 cm painting is 300dpi,  but if your scanner has good sensors and you are going to use your image for web use you can go for 150 dpi without any serious lose of quality.

If you want to scan a larger than 56×76 cm paper, then go for even lower resolution, less than 150 dpi for editing reasons. ( larger painting, more scanned parts that have to be smaller in their size).

I’m going to scan one of my night skies ( that needed either way to rescan it ).

My painting is 56×76 cm I need it for web use and to keep a HR copy just in case so I’ll scan at 150 dpi.

Here are the settings on my scanner’s program.

Resolution at 150 dpi
I have set it to sequence scanning which means that I don’t have to use the mouse in order to start the scanner but it will scan by pressing its button. ( very convenient feature for continuous scanning).

Below is my set up on the floor.
Scanner attached to the pc on the floor, sketchbook, micro fiber cloth to clean the glass and a pencil.

Place your painting on the edges of the glass of your scanner. This will ensure that your scan parts will be straight and ready for assembling on GIMP.
It doesn’t matter with which edge of the painting you’ll start.


Place the A4 carton or sketchbook ( whatever you’ll use) on the top of the painting and be sure to align it with the scanner’s glass.

Mark with your pencil the edge of the A4 part and then the top edge too. Your are going to need these marks as guides for scanning the rest of your painting.

Press the button to start scanning ( or do it with the mouse)

After the scanning of the first part is finished, proceed with the part of the painting that is next to it.

Pay attention now. In order to be easy to assemble these parts in the end, you are going to need a bit of overlap clearance. In other words a piece of the first scanned part existing also on the second scanned part.
In order to achieve this overlaping between the scanned parts you’ll have to place your painting in such a position that the mark that you did with the pencil will be a bit further inside in the scanning area.

Look at the picture below.

Align the edge of the paper with the edge of the glass. This will allow the second part to be completely parallel with the first part.

Place the A4 sketchbook on this and scan.

Do the same for the rest of the parts always taking care to have a bit of overlap between the scanned parts.

When you’ll finished the scan sequence on the lower part of your painting, turn the painting upside, align the opposite corner and start scanning with the same method the upper part of your painting.
You don’t have to care for the orientation of the scanned parts you can rotate them later on. Take care though to scan at the same direction. From left to right at each side of the painting.
The upper edges will overlap either way because the smaller (56 cm) side of full size watercolour papers is smaller than
two large sides of the A4 size paper, ( or scanning surface).

It took me about 3 minutes to scan the whole painting in 8 parts. Mine are in a .png format. I will convert to .jpg after finishing the editing.

Now that we finished with the scanning we can proceed with the editing. I’ll post here countless screenshots in order to see how is done, the actual editing though it will not take you more than 10-15 minutes.

It was impossible to take a screenshot while I was using the menu on GIMP so I’ll write what tools I used and what I did each time before each image.

Go to the folder that your scanned parts are.

Pay attention now. As I scanned my painting in a landscape position but the final image has to have a portrait orientation, I will assemble the parts in two vertical rows connected in between them.

So the first four parts of the painting have to be rotated counter clockwise and the rest four clockwise.

Open the first part with GIMP.

The signature has to be at the lower left corner.

Go to menu Image/Transform/Rotate counter-clockwise ( it has arrows that shows the direction) Click there and GIMP will rotate your image.

Look below

Remember to do that at each of the first four parts.

Now that we rotated the first part we have to create a new file on which will do the assembling.

Go from the menu: File/New

The size now of the new file has to be large enough in order to have plenty of space to place there all the scanned parts.

I calculated that I need a surface that will be 4000×6000 px.
( 8 parts of 1754×1272 px size with enough clearance to move them up and down. Whatever is not needed will be cropped at the end).

My new file is ready for use

I can zoom in and out from the drop down menu on the bottom of the new file.

Now go to the first part opened right click on it and select

Edit/Copy

Move on the New File ( the blanc one) and with right click select

Edit/ Paste

Here is the first part copied on the new file.

Select on the tool bar the Move Tool ( see on the image above were this is).

Drag with right click the image and place it on the lower left corner.

Open the next scanned part.
Rotate ( counter clock wise as shown above).

Pay attention now we are going to use the overlap thing.
As the scanner tends to distort or create shadows on the edges of the scanned image will have to crop the side that will assemble. Otherwise will create visible lines that will be difficult to remove in the end.

So will have to crop the side that will attach, ( and any side that will touch the rest of our new image).
In this case I crop the lower side.

See below. Go to the window that you rotated your second part. Select the Crop Tool on the tool bar on the side

And then I crop the lower side of the second image.

After you select which area you want to leave out, right click in the middle of the image and your image will be ready for copying on the new file.

Proceed as previously.
1/Right click on the image, select Edit/ Copy
2/Go to the new file right click and select Edit/ Paste.
3/Select the Move Tool from the tool bar to move your second part somewhere close to the first part and then zoom in to accurately bring the two parts together.

Hold your image with right click and move it until it matches accurately and overlaps the existing first part.

See below on my painting the parts that will bring together by overlapping them. I’ll make sure that the dots will be the one over the other. ( try to find a detail that will guide you in order to do so).

When you’ll find the right position go on the menu and click:

Image/Flatten Image

This will fix the position of second part in place. If you are not satisfied with the result then press Control Z on your keyboard to go back to the move option and correct the position.
You’ll have though to use again the Flatten Image in order to fix again its position.

Check now below how my two assembled parts look like.
Not a single line on the edges, absolutely perfectly assembled.

Two parts assembled till now.
We proceed with the same method until will finish the first row.

Remember to rotate your images and crop the sides that will overlap until you finish with this row of four images.

Here is the finished first row.

And now will start the next row that will attach it simultaneously on the already existed.

As I scanned in a landscape orientation and I turned the painting upside down in order to scan the other part of the painting, I’ll have to start my next raw of vertical assembling from the last of the rest four images ( in order to start again from the bottom).

(This doesn’t have to be a rule. If you assemble a landscape image and you have scanned in landscape orientation you don’t need to start from the last scanned part. )

The rest four parts have to be rotated clockwise this time. Don’t forget to rotate them at the beginning.

Load the last of the parts ( the 8th one/ check the number that the scanner gave to each file) rotate it and now pay attention.

You are going to crop the side that will touch the already assembled first row. As this was the last of the images and you probably includes some of the carton or the sketchbook you used during the scanning, it will be a good idea to crop this too. So look below what I cropped.

Same as previously I did right click: Edit/Copy
then I’m going to the new file and with right click I paste the image there. This time at the lower right corner.

Select move tool, zoom in to be easier for you to put it in place then go Image/ Flatten Image to fix its position.

Load the next part on Gimp ( the 7th one).
Will have to crop the edges that touch the rest of our assembled image.

After croping copy and paste at your new file.

Fix its position.

Next part ( 6th of the scanned)
Don’t forget to rotate.

The third part it was a bit tricky to crop for this particular painting. See below why.

I have the clearance at the last part to do so.

I proceeded with the last one and here is my assembled image complete.

Next you have to fix them permanently in position and get rid of the overlapped but not visible parts.
Go on the menu and select Image/Merge Visible Layers.
Click to discard those that are not visible any more ( less data on your file and so smaller final size). I have already cropped the blanc space of the new file.

Final Stage.. A bit of colour editing at our final image.

A bit of colour and brightness editing ( needed on my scanner because it tends to over do it with its scanning bar light).
In my image a change up to 0,81 it is ok but you have to play a bit in order to find what works better with your scanner on your pc. I change the levels of the whole painting in order to get closer to the original.

And a bit retouch on the brightness.

Ready.
Export the existing image as a .png file at Full Resolution to have it for future use. Mine end up to be 3266×4444 px at its HR version.
A sample on how it looks like a part of it at 100% size.

Then go to Image/Scale Image in order to change its size.

I made its longer side 800 px ( for web use) and I exported this file as a jpg with lower resolution.

Don’t save the last change of size ( you have exported the file as jpg after all) so close GIMP and if it gives you a warning about discarding recent changes discard them either way.

Here is my painting finished.

And that’s all folks. If you have any questions you can ask via Disqus on the comments section bellow.

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