17 декабря 2015

                    Using GIMP for pixel art
                         by Hippiman

After an article in ACNews #61 about conversion of RGB gfx into
the palette of ATM Turbo и ZX Evolution (briefly "ATM palette")
using Photoshop, some of the readers might ask about the same,
but with GIMP. The conversion process itself in GIMP doesn't
differ much from Photoshop, so a duplicate article about it
would not be interesting. So I decided to extend the topic and
write an overview about using GIMP for pixel art.

                             Drawing

The tools like pencil and fill are apparent and like those in
Photoshop, so I can skip them. I must mentionSelect by color 
tool [something like "Magic wand" in Photoshop, if you uncheck
"adjacent pixels" - Ed.]. As you can guess from the name, it 
allows to select the areas of given colour with certain
threshold. This tool is versatile, it can be used for finding
the areas with some colour while conversion, or for write
protection of some areas.

Here are few reminders that can speed up your working process in
GIMP: 

  - Brush size is changed with hotkeys "[" and "]".
  - Foreground and background colours can be swapped with "X"
key.
  - You can pick up a colour without using the dropper tool,
just hold "Control" and click on image (if any of drawing tools
is active).
  - Hold "Shift" to draw a line from the last drawn pixel.
  - Quick zoom on numpad.
  - Hold "Control" and scroll the mouse wheel to zoom.
  - You don't need scrollbars to navigate a big image that
doesn't fit on screen. Just hold middle mouse button and move
the image.

This is the base. Also try to learn the hotkeys for the most
used tools. The work will go faster.

One more important thing for pixel art is grid. You can show it
on screen withView->Show Grid. The default step is 10x10
pixels. You can change it to more common 8x8 or 16x16 in menu
Image->Configure Grid. There is more interesting in that dialog:
for example, grid colour - very useful function for editing dark
images.

                          Palette work

Let's proceed to the main thing. The palette can be found in the
top-right corner of screen, inColourmap block. You better
detach it from there by dragging, to make it a separate window.
Or a separate panel. Or in case of layer work you must switch
pages constantly. (Any floating window can be attached to any
panel if you drag it by tab header.)

Double click any colour to open the colour edit dialog. After
changing the palette colour the image will change automatically.
All the pixels with the same colour index will change colour.

If you select a foreground colour not from the palette,
drawing pixels will have the closest possible palette colour.

If you paste an RGB image from clipboard to a project with
palette, then RGB image will be corrected automatically into the
current palette without dithering.

                How to convert images to palette

If you have a ready RGB image, the conversion is simple. Just
open the dialogIndexed Colour Conversion, menu
Image->Mode->Indexed.

This dialog includes:
  -"Generate optimum palette" - allow the program decide what
colours to include in the palette. You define the"Maximum
number of colours".
  -"Use web-optimized palette" - as you can guess from the name
[looks the same as in Photoshop: palette with entries 
#00/#33/#66/#99/#cc/#ff - Ed.]. 
  -"Use black and white (1-bit) palette" - converts the image
to monochrome 2-colour b/w palette.
  -"Use custom palette" - converts to given palette that you
can prepare in advance.
  -"Remove unused colours from colourmap" - to remove the
colours used in no pixels, or not.

The most usable for our needs are the 1st and the Чth:
"Generate optimum palette" and"Use custom palette". I recommend 
to always have a ready palette file with ATM palette [it
corresponds to "Uniform" 64 colour palette in Photoshop - Ed.]. 

More options follow that define the dithering:
  -Colour dithering - select the method of dithering.
  -None - no dithering. Each pixel is changed to the closest
possible palette colour.
  -Floyd-Steinberg (normal) и Floyd-Steinberg (reduce colour
bleeding) - usual error diffusion.
  -Positioned - fixed dithering. It is simpler than
Floyd-Steinberg but looks better in pixel art.
  -Enable Dithering on transparency - apply dithering to alpha
channel.

For small images, fixed dithering is better, or no dithering at
all. Unfortunately I haven't found additional diffusion controls
for GIMP. The only additional variant you can use is a shareware
plug-in for Photoshop named "XiColorDither" with 30-day trial
period ( Photoshop plug-ins can be used in GIMP with PSPI
add-on). Anyway, automatic dithering looks dirty on small
images. Sprites can't be converted without hand correction even
with XiColorDither. More or less nice result appears from
resolution 320x200 or more.

Now let's see how to convert to ATM palette using this dialog.

 1. Do "Generate optimum palette". Guess "Maximum number of
colours" by hand, depending on image, trying to acquire decent
picture with minimum colours. That's usually 60 to 70 colours.
 2. Try to convert picture back to RGB, then again in ATM
palette prepared in advance, with"Use custom palette".
GIMP will correct all the colours according to given palette,
picking the closest colour.
 3. If the picture has changed a lot with ATM palette, undo
the change and try to adjust the problematic colours by hand.
 4. If there are <= 16 colours, proceed to hand correction of
final picture.
 5. If there are more than 16, find similar colours to "join"
them. I usually do it the following way. I look at the palette,
find similar colours, and change one of them for some unused
colour (such as magenta). (It would be better if GIMP had
automatic highlight for pixels with given colour clicked in
palette. But there is none.) If the area of this colour is too
small, I just change the colour to similar one. So the palette
has identical colours.
 6. When there are no colours that can be joined, try to remove
some colour by mixing two others. For example, if there is an
orange area, it can be changed to chequered pattern of red and
yellow. How to do this:
  - Create one more layer. Draw a small filling pattern in it.
For example, 4 pixels: 2 red and 2 yellow. Copy this example in
clipboard and hide the layer.
  - Using"Select by color" select all the areas with needed
colour.
  - Select"Bucket fill tool".
  - In"Fill type" select "Pattern fill". Press on highlighted
textured square and select the top left corner in the pulldown
list. This is our filling pattern.
  - Don't forget to check"Fill whole selection", or else only
the clicked area will be filled.
  - Fill the needed colour. Then look at the result and correct
it by hand.
 7. After all manipulations the palette will reduce
significantly. To update it, convert image to RGB:
Image->Mode->RGB, then back into palette:Image->Mode->Indexed, 
with optimal palette.
 8. If there are still more than 16 colours, return to 4.

As a variant for more good-looking result, you can reduce the
colour number beforehand, withColours->Posterise. In the dialog
you can define the level of posterisation. Use the result like
the original picture. Experiments show that 256-colour image
with sprites, converted to 16 colour ATM palette with
posterisation of level 3, looks better than the same without
posterisation.

                       Saving the palette

When you draw graphics for a game screen, you need it all in one
palette. The simplest way to do this is to save the palette of
first image in file, then draw the rest of gfx in this palette.

The algorithm:
 1. Colors->Map->Set Colormap
 2. Press Default, it opens the window Script-fu: palette
selection.
 3. Scroll the list to the top and find the current image
colour map there.
 4. Click this colour map with right mouse button. Select
Duplicate palette.
 5. Rename the copy of current colour map in meaningful way.
 6. That's all - now the palette can be accessed from any
image, in all dialogs.

The palettes are usually saved in
C:Users%username%.gimp-2.8palettes.
This path can be changed in menu
Edit->Preferences->Folders->Palettes.
Palette file is a simple text file that can be edited by
notepad. Example of the contents:

GIMP Palette
Name: Colormap of Image #1
Columns: 8
#
255   0   0#0
210   3  16#1
 6   8   4#2
177 183 191#3
... 
208  97 145#255

If needed, all the palette can be edited by hand.

                          Saving as BMP

After all the manipulations with the project you might want to
get a readyBMP to use elsewhere. For some unknown reason, in
newer versions of GIMP"Save"/"Save as" only saves in GIMP's
native format -XCF. To save in other formats, use
"Export"/"Export as".

The following is very important!!! In dialog"Export Image as 
BMP", open"Compatibility options" and check "Do not write 
color space information". Or else the savedBMP might be
incompatible with some programs. For example, EvoSDK can't
find a palette in this kind ofBMP and treats it as black and
white.

                             * * *

GIMP is a good free program. It has enough function for most of 
tasks except the professional photo processing. I tried to
describe the most needed things. Of course, the possibilities
are a lot wider than just drawing colour pixels, but this is
good for start.

In the pack of this issue you can find the mentioned ATM
palette with 64 colours