[Gimp-user] global color changes

[whburling <forums gimpusers com>]

> First turn off the active selection: Select -> None
> With such a tiny image, the chequer "transparency" pattern gets in the
> way. Add a layer underneath and fill with a colour. You want that
> anyway for a new background.
> It is there if you look. Probably you saw some Gimp 2.8 examples, Gimp
> 2.10 requires the user to set transparency first. White is the default
> colour.
>
> Layer -> Transparency -> Add Alpha channel then Color -> Color to
> Alpha. That menu entry is down toward the bottom.
> Black lines to white lines in the layers once they have transparency.
> Use Color -> Invert
>
> some screenshot examples, https://i.imgur.com/U1hKYRa.jpg sorry this
> is a bit terse, but I am packing in for the day.
>
> rich: www.gimp-forum.net


Rich....I have attempted to implement what I thought was your suggestion.
I have stated it below. In trying to make my mask....I encountered a problem.
I could not select a portion of my mask pattern to repeat by pasting and
anchoring.
Can you tell me what I might be doing incorrectly?

Goal
Create a test pattern of 24 pairs of vertical lines (one  black, one of four
colors) for an electronic sign 112 pixels wide and 48 pixels high.  The color
lines of the pair are Red or Green or Blue, or White.

If I can achieve this goal, I can then make other repetitive test pattern
Strategy                
Create four base layers each one of the four colors.

Create a layer, whose use is as a mask, which can be imposed over the above base
layers creating the test pattern.
I am not using a predefined GIMP “mask” as masks are associated with a single
image layer.
By using a layer whose use is as a mask, one such layer can be a mask for all
base layers

Somehow flatten a base layer and mask to create a test pattern that can be saved
as a file for each test pattern.  I am not sure how to preserve the base layers
and mask in achieving the above. Perhaps I save four additional copies of the
file with the base layer and mask. Each of the four additional copes can then be
flattened and the flattened file saved as the file name and type that can be
sent to the sign controller as a test pattern.

Methodology     
Create a canvas
        Initialize
                Define foreground color as Magenta

        File -> new ->  canvas size
                Width = 114  (2 pixels wider than sign display area)
        `              height = 50   (2 pixels higher than sign display area)
                             Units: px
                Advanced Options
                        Color space RGB
                        Spatial resolution 0.05 pixels/mm
                        Fill with foreground color
                        Note: “layer used to detect errors”
        Implicit (settings I did not explicitly set)
                Label of layer predefined by application as “Background”
                Layer is in view (eye icon showing)
                

Create view of canvas
        View->zoom->fit image in window


Begin Image Construction
        Initialize
Define “foreground” color Red (implicitly defining paint in bucket), 

Create a base layer, 
create a new layer. 
> From new layer dialog box, select “fill with foreground color”.   
Select bucket fill tool. 
select entire layer(implicitly already done), 
click in layer (activating bucket fill tool) thus filling layer with Red. 
        Right click layer
        Select edit layer attribute
                Select switch that locks pixels
                Label layer as “Red On-Pixels”
        Repeat for all four colors (R,G,B,W) with the correct  color in label

        Create  a mask
Do NOT use the predefined GIMP mask as such masks are associated with a
specific
image layer. Instead create a single mask that can be used with all base layers.
This is achieved by creating a new transparent layer and with a pencil make
black vertical lines one pixel wide. The black lines define the off pixels. The
transparent region allows the
underlying Base layer to define the color of the on pixels.

        PROBLEM:
To create the above layer, I want to draw with a pencil a few vertical lines
with a pencil using black as the pencil color, then select the few lines , copy,
paste, and anchor. Then repeat the process with a larger set of lines until I
finally have an entire layer filled with vertical lines

I can draw the pencil lines. I can’t select and then paste them into the layer.
The error message:  “Cannot intersect with an empty selection”.

I do not know how to interpret that error message and hence have no idea what to
do differently.

My attempt to understand the above error message is that the black lines I put
on the transparent layer are not interpreted as having any physical reality.

-- 
whburling (via www.gimpusers.com/forums)