I will explain the steps you need to take using a photo of one of my own pieces as the example. The piece has been shot on a perfectly white background (a sheet of white plexiglass) and in a tent-like softbox. To make sure the white was not looking white I did not set up the lights I am normally using and the softbox was placed under a skylight giving enough light to see the softbox's frame reflections on the surface of the plexiglass sheet.
|My perfectly imperfect original photo|
Step 1 - adjust curvesYou will find the 'Curves' command in the 'Colors' menu:
This is what you see after opening the 'Curves' tool:
Drag the top end of the line in the graph to the left until you go past the peak of the gray graph. At this moment your background should be already looking quite white. Click OK.
Experiment with moving the end of the line to the left further and further. This will help you understand how the things work (in short: the more to the left you drag the point, the brighter the picture is becoming. Some photos will need more adjustment than others).
*** If your item is white or very light it is very likely that after adjusting curves it won't be clearly visible any more, since what we are doing in this step affects not just the background but all the light areas of the photo.
Step 2 - duplicate the background layer
You will find the 'Duplicate Layer' command in the menu that shows after right-clicking the name of the layer on the layers' list.
After you click the 'Duplicate Layer' command, a copy of your original layer will be created. It will have the same name as the original with the word 'copy' added at the end. I will be referring to this layer as to the threshold layer.
Step 3 - reduce the image to two colors and set the threshold
Make sure the threshold layer is highlighted at this moment.
Go to Colors - Threshold and click it:
Your photo will turn black and white and you will see a graph with a small arrow slider under it:
Drag that arrow all the way to the right, there will be no gray on the graph any more. Click OK.
Looking at your photo now you can tell what areas of your photo are perfectly white and which are not, even though on the 'normal' version of the photo they might seem to be such.
Now all you have to do is get rid of all the black spots in the places where the photo should be white.
*** If you haven't moved the curves' line far enough to the left your image will be completely black and you won't be able to move to the next step. Try to go back and adjust the curves again and if it doesn't help go back even more and brighten the photo. If you still cannot recognize the outlines of your item on the threshold image it means that your photo is far too dark and you won't be able to get a perfectly white background. It is up to you if you keep it as it is after adjusting the curves only or you take another photograph of your item which hopefully will be brighter and you can work with it.
Step 4 - clean up the background
Now is the time to get rid of all the imperfections of the background. To do this stay on the new layer and pick the brush tool.
Set the foreground color to white (RGB 255-255-255) and adjust the size of the brush to a size that will be comfortable for you. Remember that you have to get rid of the gray areas on the background without touching the edges of your item!
Start to paint white over gray:
This is how the photo should look like at the end:
Step 5 - apply the clean background to your photo
The last thing you will have to do is copy the perfectly white background from your threshold layer to the original one.
Go to Select - By Color. In the Tool Option make sure that none of the boxes are checked, the Threshold value is set to 0.00 and Value is chosen from the Select by list. Then click anywhere on the white area of the threshold layer (you have to do it only once even if you have more than one patch of white) and press Ctrl+C to copy your selection:
Turn off the visibility of the threshold layer and activate the original layer for editing. Click Ctrl+V to paste your white background:
You will see a new item show on the layer list, this is your selection pasted. Right click on it and choose 'New from visible' from the list - this will create a new layer called Visible. Now you can delete all the other layers from the list:
Voila. The background is done, now you only need to fine tune the levels and contrast of the photo, and that's it. Here is my finished photo. No seams on the white page, yes?
So, this is how you can tune your background in Gimp. It might look complicated, but in fact it is very simple and editing one photo shouldn't take more than a couple minutes.
In my next post I will explain how to get white backgrounds in Photoshop (it will be a very short coverage, since PS is so much easier and nicer to work with than Gimp). I will also give some tips for shooting photos with white backgrounds - including the use of a light-table.