(02-03-2017, 03:45 PM)Ofnuts Wrote: Thinks of your image build by stacking sheets of glass on which you paint things. Each sheet is a layer.

The Layers dialogs shows you how they are stacked (so you can change the stack order). It also tells you which are visible (ie, actually in the stack) By clicking on an eye icon you can make a layer visible/invisble (ie, temporarily remove it from the stack).

The RGB channels are there mostly for display. You can also copy them to plain channels by dragging them down or enable/disable them, but this has little purpose. The main purpose of the channels dialog is to manage the channels you create (by saving selections, mostly).

The alpha-channel is a fourth image component (besides the R,G,B ones) that controls the opacity of the pixel. You don't choose a color to be transparent. A pixel is transparent because its alpha value is 0, whatever the R,G and B values. A pixel can also be partially transparent if its alpha is less than 1.

Transparency is edited implicitly or explicitly. When you use the eraser or use Edit>Clear, the alpha value is set to 0 (if you are using a soft brush fro the eraser, the alpha can have an intermediate value). If you want to edit the alpha channel explicitly, you use a layer mask. This is more or less a companion grayscale layer on which whatever you draw/paint is interpreted as the alpha channel of the "parent" layer.

Thank you for responding. You say

'The alpha-channel is a fourth image component (besides the R,G,B ones) that controls the opacity of the pixel. You don't choose a color to be transparent. A pixel is transparent because its alpha value is 0, whatever the R,G and B values. A pixel can also be partially transparent if its alpha is less than 1.

If I cannot choose to make a colour transparent. How would I make the background transparent?

(02-03-2017, 05:45 PM)anon_private Wrote: Thank you for responding. You say

Quote:'The alpha-channel is a fourth image component (besides the R,G,B ones) that controls the opacity of the pixel. You don't choose a color to be transparent. A pixel is transparent because its alpha value is 0, whatever the R,G and B values. A pixel can also be partially transparent if its alpha is less than 1.

If I cannot choose to make a colour transparent. How would I make the background transparent?

(02-03-2017, 06:08 PM)Ofnuts Wrote:

(02-03-2017, 05:45 PM)anon_private Wrote: Thank you for responding. You say

Quote:'The alpha-channel is a fourth image component (besides the R,G,B ones) that controls the opacity of the pixel. You don't choose a color to be transparent. A pixel is transparent because its alpha value is 0, whatever the R,G and B values. A pixel can also be partially transparent if its alpha is less than 1.

If I cannot choose to make a colour transparent. How would I make the background transparent?

If you want to delete a specific color and replace it with transparency, two completely equivalent techniques:

1) Colors>Color to alpha and pick up the color of the background

or

2) Bucket-fill the image with the background color after setting the bucket-fill tool in  "Color erase" mode

Both techniques require an alpha channel in the layer: Layer>Transparency>Add alpha channel

Both techniques have a side effect, if there are parts of your subjects with colors close to the color of the background, they
will become (partially) transparent as well. So before applying the above techniques you can restrict their action to the background and also
to the very important anti-aliasing pixels:

- using the wand, select the background
- Select>Grow by two pixels so that the selection includes the border pixels
- Apply Color-to-alpha or Color-erase as above.


Thank you.

After making adjustments to an image can I view it as it would be seen on a webpage, or, must I upload it to a site to complete the test?

Best wishes

(02-04-2017, 02:53 PM)anon_private Wrote: Thank you.

After making adjustments to an image can I view it as it would be seen on a webpage, or, must I upload it to a site to complete the test?

Best wishes

Ps. I am using gimp 2.8.10 under kubuntu (14.10). Would you advise installing version 2.8.20, or, is there little difference between the programmes? My pc is a Dell Dimension E520, 1GB RAM, Pentium D (2.7GHz).

• The file has been reencoded by the website.  This is not uncommon for large websites (Facebook...) and of course usually the image takes a beating in the process. Just save the image again from the web page and compare to your original file.
  
• The HTML that includes the image enforces a size that is slightly different from its native size making the browser rescale the image on the fly. This is confirmed either by inspecting the HTML source or by making the browser open the image all by itself, which should restore it to is original size and quality.
  

(02-04-2017, 03:29 PM)rich2005 Wrote: It is certainly worth updating, very easy to do with (k)ubuntu. You need to add a ppa

see: https://launchpad.net/~otto-kesselgulasch/+archive/ubuntu/gimp

Still Gimp 2.8.18 I expect Gimp 2.8.20 will be there soon.

(02-04-2017, 06:11 PM)Ofnuts Wrote:

(02-04-2017, 02:53 PM)anon_private Wrote: Thank you.

After making adjustments to an image can I view it as it would be seen on a webpage, or, must I upload it to a site to complete the test?

Best wishes

Ps. I am using gimp 2.8.10 under kubuntu (14.10). Would you advise installing version 2.8.20, or, is there little difference between the programmes? My pc is a Dell Dimension E520, 1GB RAM, Pentium D (2.7GHz).

If it doesn't look the same way in Gimp than on the web site, it it is usually that the web site has done something to it. Just load your local file in your browser (File>Open in Firefox, or drag to Firefox/Chrome). If it OK there on not on the web site then either:

• The file has been reencoded by the website.  This is not uncommon for large websites (Facebook...) and of course usually the image takes a beating in the process. Just save the image again from the web page and compare to your original file.
  
• The HTML that includes the image enforces a size that is slightly different from its native size making the browser rescale the image on the fly. This is confirmed either by inspecting the HTML source or by making the browser open the image all by itself, which should restore it to is original size and quality.
  

Always better to have a few less bugs.... See Rich's indications on how to include Otto's PPA in your software sources. This provides updates to Gimp but also to the rather useful gimp-registry filters. However, with 1GB your computer is on the small side, especially if you run KDE.

(02-05-2017, 01:53 PM)anon_private Wrote:

(02-04-2017, 03:29 PM)rich2005 Wrote: It is certainly worth updating, very easy to do with (k)ubuntu. You need to add a ppa

see: https://launchpad.net/~otto-kesselgulasch/+archive/ubuntu/gimp

Still Gimp 2.8.18 I expect Gimp 2.8.20 will be there soon.

Thank you for responding.

I note from the ppa page that it is for ubuntu. I assume that kubuntu is also appropriate.

I note on the bottom of the ppa page:

'Adding this PPA to your system'

'You can update your system with unsupported packages from this untrusted PPA by adding ppa:otto-kesselgulasch/gimp to your system's Software Sources.'

It does say 'untrusted'!

Quote:Is there a feature that creates solid coloured objects such as: spheres, arrows, lines, hexagons, etc.