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diff --git a/_posts/2018-06-23-slicing-images-gimp-python.md b/_posts/2018-06-23-slicing-images-gimp-python.md new file mode 100644 index 0000000..73ba570 --- /dev/null +++ b/_posts/2018-06-23-slicing-images-gimp-python.md @@ -0,0 +1,202 @@ +--- +layout: default +title: "Slicing and Dicing Images with GIMP and Python" +--- + +# {{ page.title }} + +Let's say you have one big image (say, a Telegram sticker) and you need to dice it into a bunch of smaller images (say, Discord emoji). +GIMP can let you do that manually, but frankly so can simpler tools. +GIMP also has powerful scripting support with Python (and also Scheme, but miss me with that) that can let us do that automatically. + +## TL;DR how do i do the thing + +1. Save your large image somewhere with a useful filename; this script will chuck `_1_1` and `_1_2` etc on the end of the existing filename. +2. Open that image in GIMP. +3. Go to the Filters menu, open Python-Fu, and hit Console. +4. Set up the width and height of your tiles. For 64x64 tiles, for example, type + ```python + WIDTH = 64 + HEIGHT = 64 + ``` +5. Paste in this big ol' block of code and let it build your tiles and print out the text you can enter in Discord to reconstitute your original image: + ```python + from gimpfu import * + from __future__ import print_function + import os.path + + def crop(image, x, width, y, height): + pdb.gimp_image_crop(image, width, height, x, y) + x_idx = x / width + 1 + y_idx = y / width + 1 + filename = pdb.gimp_image_get_filename(image) + dir, name = os.path.split(filename) + root, ext = os.path.splitext(name) + ext = ".png" + output_root = root + "_" + str(y_idx) + "_" + str(x_idx) + output_name = os.path.join(dir, output_root + ext) + layer = pdb.gimp_image_get_active_layer(image) + pdb.file_png_save_defaults(image, layer, output_name, output_name) + print(":" + output_root + ":", end="") + pdb.gimp_image_delete(image) + + image = gimp.image_list()[0] + filename = pdb.gimp_image_get_filename(image) + + for y in range(0, 512, WIDTH): + for x in range(0, 512, HEIGHT): + crop(pdb.gimp_file_load(filename, filename), x, WIDTH, y, HEIGHT) + print() + + pass + ``` + +There are two minor issues with actually using this code to convert a Telegram sticker into Discord emoji that I'll get to later. + +## The Code, Splained + +I'll walk through each bit of the code segment above and explain why it's there. + +We need the GIMP libraries, the Python 3 `print()` function (because as of GIMP 2.8.22 the GIMP console is still on Python 2), and some path manipulation functions. +```python +from gimpfu import * +from __future__ import print_function +import os.path +``` + +We're going to crop an image with an X and Y offset and a width and height. +The first step in generating the tile is telling GIMP to do the actual crop. +```python +def crop(image, x, width, y, height): + pdb.gimp_image_crop(image, width, height, x, y) +``` + +The next step is to figure out the filename for this specific tile; here we're getting an index back from the offsets and width and height. +```python + x_idx = x / width + 1 + y_idx = y / width + 1 + filename = pdb.gimp_image_get_filename(image) + dir, name = os.path.split(filename) + root, ext = os.path.splitext(name) + ext = ".png" + output_root = root + "_" + str(y_idx) + "_" + str(x_idx) + output_name = os.path.join(dir, output_root + ext) +``` + +Once we've got a filename, we can save. +For some reason GIMP's save functions all depend on both the image and the layer, and on two copies of the filename. +```python + layer = pdb.gimp_image_get_active_layer(image) + pdb.file_png_save_defaults(image, layer, output_name, output_name) +``` + +Since the goal is to reconstitute the original image from Discord emoji, we assume that they won't be renamed. +We need the Python 3 print function here to suppress any characters after the string is printed; the Python 2 `print "foo",` trick still emits a space. +```python + print(":" + output_root + ":", end="") +``` + +We might as well delete the image from GIMP. +I don't know if this actually serves an important purpose or not. +```python + pdb.gimp_image_delete(image) +``` + +We want to grab the original filename. +```python +image = gimp.image_list()[0] +filename = pdb.gimp_image_get_filename(image) +``` + +Since we defined WIDTH and HEIGHT manually earlier, now we can loop through the image. +I should probably go back in and make it grab the full image width and height, but fuck it, I don't want to. +```python +for y in range(0, 512, WIDTH): + for x in range(0, 512, HEIGHT): +``` + +I don't know if GIMP doesn't expose undo in the Python API or if I just couldn't find it, but either way we don't have undo, so we pass in a fresh copy of the image instead. +```python + crop(pdb.gimp_file_load(filename, filename), x, WIDTH, y, HEIGHT) +``` + +Since we're building up the emoji text for Discord one row at a time, we need to end the row at the end of a row. +```python + print() +``` + +This is just there so the newline after the `for` loop gets pasted successfully. +```python +pass +``` + +## The Plot Thickens + +The first issue with this approach is that Discord (at time of writing, at least) sets a total of 2.25 pixels worth of horizontal margin between emoji, so your reconstituted image will have weird stripes. +It might be feasible to adjust for these in the offsets so that the spacing isn't funky, but honestly that seems like a lot of work. + +The second, and more interesting, issue is that Discord has a 50 emoji limit on each server (at least for non-Nitro plebeians; I don't know if that changes if the server owner upgrades). +Slicing a 512x512 image into 32x32 tiles for a full size replica would generate 256 tiles, which might work if you had Discord Nitro and six different dummy servers, but nah. +Slicing into 64x64 tiles that'll be rendered at half size only makes 64 tiles, which works out nicely numerically but is still more than can fit on one server. +Unless we're clever. + +I'm not sure how well this generalizes, but for the sticker I'm working with, 16 of those 64 tiles are fully transparent, and therefore identical. +If we could detect this when slicing, we could avoid emitting 15 of those, at which point we come in nicely with 49 tiles, one under the Discord emoji limit. +But how can we detect if an image is fully transparent? + +Get histogram info for the alpha channel! +We can use something like this to count how many pixels aren't fully transparent: +```python +_, _, _, _, visible_count, _ = pdb.gimp_histogram(layer, HISTOGRAM_ALPHA, 1, 255) +``` + +So our final code can detect if each tile is fully transparent before it saves and treat all fully transparent tiles as equivalent to the very first one. + +```python +from gimpfu import * +from __future__ import print_function +import os.path + +empty_tile_name = None + +def crop(image, x, width, y, height): + global empty_tile_name + pdb.gimp_image_crop(image, width, height, x, y) + layer = pdb.gimp_image_get_active_layer(image) + _, _, _, _, visible_count, _ = pdb.gimp_histogram(layer, HISTOGRAM_ALPHA, 1, 255) + x_idx = x / width + 1 + y_idx = y / width + 1 + filename = pdb.gimp_image_get_filename(image) + dir, name = os.path.split(filename) + root, ext = os.path.splitext(name) + ext = ".png" + output_root = root + "_" + str(y_idx) + "_" + str(x_idx) + output_name = os.path.join(dir, output_root + ext) + if visible_count > 0 or empty_tile_name is None: + pdb.file_png_save_defaults(image, layer, output_name, output_name) + if visible_count == 0: + if empty_tile_name is None: + empty_tile_name = output_root + else: + output_root = empty_tile_name + print(":" + output_root + ":", end="") + pdb.gimp_image_delete(image) + +image = gimp.image_list()[0] +filename = pdb.gimp_image_get_filename(image) + +for y in range(0, 512, WIDTH): + for x in range(0, 512, HEIGHT): + crop(pdb.gimp_file_load(filename, filename), x, WIDTH, y, HEIGHT) + print() + +pass +``` + +The results are actually fairly impressive, all things considered: + + + +(that sticker is by [NL](https://twitter.com/NLDraws) and of [Pandora's Fox](https://twitter.com/pandoras_foxo)) + +But of course anyone with an ounce of sense would just upload the image so this whole project was a complete waste of three hours.
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