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I have some jpg images and I need for each of them to have a transparent background.

I have GIMP as a graphic tool.

I need to put them in a web page and I need to see just the main element and have a transparent background.

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3 Answers 3

To expand on utopicam's answer, sometimes your image might contain areas, such as shadows, that you'd want to be semitransparent. For example, let's say you have a photo shot in a lightbox, such as this nice and freely licensed picture of a Swedish wooden toy horse courtesy of Creative Tools:

A photo of a Dalekarlian toy horse in a lightbox, licensed under CC-By 2.0 by Creative Tools

In general, the first step would be to adjust the levels of the image until the background really is solid white, except for the shadows and reflections we want to keep. (You can use this technique with a gray or colored background too, but it's a bit trickier to get good results that way.) In this case, the image is already nicely adjusted, so we don't have to do anything about that.

The next step is to select the background. This was quite easy to do with the magic wand tool, although I had to fix some bits where the white stripes on the object blend with the background with the lasso tool. Also, once I had the background nicely selected, I expanded the selection by a few pixels (at full resolution) to avoid leaving a whitish halo around the object due to aliasing. (You can't see it in this scaled-down screenshot, but I really managed to do a pretty nice job with the selection, if I may say so myself.)

Same picture with the background selected

Now, here's the trick: instead of simply cutting out the background, I used ColorsColor to Alpha to change the background color (white) to transparent. (You could also use bucket fill with the Color Erase mode as utopicam suggests.) If applied to the whole image, this would make the object look semitransparent too, but since only the background is selected, the shadows become translucent while the object itself stays opaque:

Picture after applying Color to Alpha

And here's the same picture on a pale blue background, to show the semitransparent shadows and reflections better:

Same picture on a pale blue background

One extra trick I didn't show is that it's often more convenient to make two copies of the image layer, convert the selection to a layer mask on the upper one, and apply Color to Alpha to the lower. This lets you fine-tune the edges of the object by editing the mask, while seeing live how the result will look. (Tip: make a few background layers filled with different colors, so that you can easily check how the picture looks on different backgrounds.) In particular, sometimes making highlights near the edges of the object just slightly translucent can make the result look nicer and more natural, since it mimics the background reflecting off the object.

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the first step would be to adjust the levels of the image until the background really is solid white - should how to do this exactly be included for us dummies? Or should I ask a separate question on how to do that? –  hippietrail Jan 13 '13 at 1:49
1  
@hippietrail: The levels tool generally does a pretty good job: just move the rightmost input slider left until it looks good, or use the white eye-dropper. If the cutoff looks too sharp, you can switch to "Edit these settings as Curves" and smooth it out there. (Basically, what you usually want is a curve that's mostly linear, but bends a bit near the top edge to meet it gradually rather than in a sharp corner.) –  Ilmari Karonen Jan 13 '13 at 5:45

Sounds like you need to crop certain parts of the jpgs (the main figure, for example) and delete the "background". You can do so using the lasso tool.

The gimp website has lots of tutorials and explanations. Check this http://docs.gimp.org/en/gimp-painting.html#gimp-concepts-selection and this out: http://docs.gimp.org/en/gimp-tool-free-select.html

The polygonal selection creates an anchor point every time you click. Then moving the mouse pointer draws a line with a new anchor point that you can move as long as you don't click again.

You can access the Lasso Tool in different ways:

  • From the image menu bar Tools → Selection Tools → Free Select,

  • by clicking on the tool icon in the ToolBox,

  • by using the keyboard shortcut F.

To make the solid background of an image transparent, add an Alpha channel, and use the Magic Wand to select the background. Then, use the Color Picker tool to select the background color, which becomes the foreground color in Toolbox. Use the Bucket Fill tool with the selected color. Set the Bucket Fill mode to “Color Erase”, which erases pixels with the selected color; other pixels are partially erased and their color is changed.

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Using bucket fill instead of contour selection

Note: This tutorial is also available in PDF.

_La trahison des images_ by fellow countryman René Magritte; partly vandalised for the purpose of this tutorial.

Add an alpha channel

Some image types lack a transparency channel; JPG for example. If this is the case, add an alpha transparency channel. This is done by selecting Layer → Tranparency → Add Alpha Channel.

Bucket fill with colour

The next step is filling the area that should become transparent with a plain colour first. Select the bucket fill tool or hit [Shift]+[B]. Within the Bucket Fill toolbox options, select Fill transparent areas only when necessary. Do select BG colour fill and Sample merged and run a few tries while adjusting Threshold for best results. In between, hit [Ctrl]+[Z] to undo.

Some colour mixing will be inevitable and even desired. (In this respect, Adobe Photoshop is no different.) Hence, choose a colour which is compatible with the background on which the resulting picture will eventually be used. Click on the background colour rectangle in the upper half of the Tool Options toolbox window to change the background colour. Here, I used f3f3e9 as a background colour for plain colour filling. However, one can also use the eyedropper tool to pick a colour from anywhere on the desktop.

<code>Bucket Fill</code>:  Select <code>Fill transparent areas</code> only when necessary. Do select <code>BG colour fill</code> and <code>Sample merged</code> and run a few tries while adjusting <code>Threshold</code> for best results. In between, hit <code>[Ctrl]+[Z]</code> to undo.

<code>Bucket Fill</code> applied. Choose a fill colour which is compatible with the background on which the resulting picture will eventually be used. This is because some colour mixing is inevitable and even desired. Here, I used <code>f3f3e9</code> for plain colour filling.

Select by colour

The right thing to do now is Select → By Colour. It might be tempting to perform Colour to Transparency, however this would also change semi-transparent areas within the desired image area. This time around, do select Fill transparent areas as well as Sample merged. For this particular example I chose not to apply Antialiasing nor Feather edges. Nonetheless, do experiment with these settings and choose what looks best. Again, run a few tries while adjusting Threshold for best results. In between, hit [Ctrl]+[Z] to undo.

<code>Select by Colour</code>:  This time around, do select <code>Fill transparent areas</code> as well as <code>Sample merged</code>. Again, run a few tries while adjusting <code>Threshold</code> and experiment with <code>Antialiasing</code> and <code>Feather edges</code> for best results. In between, hit\ <code>[Ctrl]+[Z]</code> to undo.

<code>Select → By Colour</code> applied

Subtract from the selection

At times, it proves difficult to get the colour selection exactly right. Some of the desired image area might be selected as well because the colour matched. In this case, and barely visible, nine desired pixels in the middle of the pipe were also selected for deletion. When this happens, use [Ctrl]+Drag with a Select tool of your choice to select a subtractive selection area.

Use <code>[Ctrl]+Drag</code> with a <code>Select</code> tool of your choice to select a subtractive selection area.

The desired transparency area is now selected.

Delete

Once the desired transparency area is selected, simply hit [Delete].

Once the desired transparency area is selected, simply hit <code>[Delete]</code>.

Final result

Finally, export the transparent image with File → Export As... to a file format which supports transparency. That would be preferably PNG, but GIF would work as well.

The final result

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You may want to grow selection by 1px to remove white edges (Select > Grow > 1 > OK) –  MonoThreaded yesterday

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