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add primitives for vertical- and horizontal-symmetry #1862

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9a467e4
add primitives for vertical- and horizontal-symmetry (needed in the l…
schanzer Mar 27, 2026
81cb2ab
expose new primitives
schanzer Mar 27, 2026
d67b439
Revert "add primitives for vertical- and horizontal-symmetry (needed …
schanzer Mar 27, 2026
a33f7c7
re-do the changes without all the whitespace bullshit
schanzer Mar 27, 2026
df7657c
rename primitives
schanzer Mar 27, 2026
912d8a6
fix function names for test suite
schanzer Mar 27, 2026
a8e058d
integrate improvements from @blerner
schanzer Mar 27, 2026
616f18c
attempting to implement @blerner s suggestion
schanzer Mar 28, 2026
65d19a8
Emmanuel learns how the annotation system works
schanzer Mar 28, 2026
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2 changes: 2 additions & 0 deletions src/arr/trove/starter2024.arr
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Original file line number Diff line number Diff line change
Expand Up @@ -235,6 +235,8 @@ provide from I:
underlay-align-list,
underlay-list,
underlay-xy,
vertical-symmetry,
horizontal-symmetry,
wedge,
x-center,
x-left,
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49 changes: 49 additions & 0 deletions src/js/trove/image-lib.js
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Expand Up @@ -1759,6 +1759,52 @@

LineImage.prototype = heir(BaseImage.prototype);

var verticalSymmetry = function(img) {
var width = img.getWidth();
var height = img.getHeight();
var halfW = Math.floor(width / 2);
if (halfW === 0 || height === 0) { return 1; }
var canvas = makeCanvas(width, height);
img.render(canvas.getContext("2d"));
var data = canvas.getContext("2d").getImageData(0, 0, width, height).data;
var sumSq = 0, count = 0;
for (var y = 0; y < height; y++) {
for (var x = 0; x < halfW; x++) {
var i1 = (y * width + x) * 4;
var i2 = (y * width + (width - 1 - x)) * 4;

@blerner blerner Mar 27, 2026

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I hate magic formulas like these :( Though I do understand it, I kinda wish we had

const getPixel(data, x, y, channel) => data[((y * width) + x) * 4 + channel];

And then you'd have var d = getPixel(data, x, y, ch) - getPixel(data, width - 1 - x, y, ch), which to me reads a lot more cleanly. OTOH, you're writing this in raw JS because of performance, so you could do

const basePixelIndex(x, y) => ((y * width) + x) * 4;

and then have

var i1 = basePixelIndex(x, y);
var i2 = basePixelIndex(width - 1 - x, y);

which saves two whole multiplications per loop ;-)

for (var ch = 0; ch < 4; ch++) {

@blerner blerner Mar 27, 2026

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What is the actual similarity formula being used in these two functions? I think, if I'm understanding correctly, that it's the root mean-squared error over every pixel and every color channel? Probably worth a clarifying comment somewhere...

var d = data[i1 + ch] - data[i2 + ch];
sumSq += d * d;
}
count += 4;
}
}
return 1 - Math.sqrt(sumSq / count) / 255;
};

var horizontalSymmetry = function(img) {
var width = img.getWidth();
var height = img.getHeight();
var halfH = Math.floor(height / 2);
if (width === 0 || halfH === 0) { return 1; }
var canvas = makeCanvas(width, height);
img.render(canvas.getContext("2d"));
var data = canvas.getContext("2d").getImageData(0, 0, width, height).data;
var sumSq = 0, count = 0;
for (var y = 0; y < halfH; y++) {
for (var x = 0; x < width; x++) {
var i1 = (y * width + x) * 4;
var i2 = ((height - 1 - y) * width + x) * 4;
for (var ch = 0; ch < 4; ch++) {
var d = data[i1 + ch] - data[i2 + ch];
sumSq += d * d;
}
count += 4;
}
}
return 1 - Math.sqrt(sumSq / count) / 255;
};

var colorAtPosition = function(img, x, y) {
var width = img.getWidth(),
height = img.getHeight(),
Expand Down Expand Up @@ -2038,6 +2084,9 @@
colorBlue: colorBlue,
colorAlpha: colorAlpha,
colorString: colorString,

verticalSymmetry: verticalSymmetry,
horizontalSymmetry: horizontalSymmetry,
}
);
return RUNTIME.makeJSModuleReturn();
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2 changes: 2 additions & 0 deletions src/js/trove/internal-image-typed.js
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Original file line number Diff line number Diff line change
Expand Up @@ -156,6 +156,8 @@
"image-baseline": ["arrow", ["Image"], "Number"],
"image-pinhole-x": ["arrow", ["Image"], "Number"],
"image-pinhole-y": ["arrow", ["Image"], "Number"],
"vertical-symmetry": ["arrow", ["Image"], "Number"],
"horizontal-symmetry": ["arrow", ["Image"], "Number"],
"name-to-color": ["arrow", ["String"], "OptColor"],
"color-named": ["arrow", ["String"], "Color"],
"empty-image": "Image"
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2 changes: 2 additions & 0 deletions src/js/trove/internal-image-untyped.js
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Expand Up @@ -146,6 +146,8 @@
"image-baseline": ["arrow", ["Image"], "Number"],
"image-pinhole-x": ["arrow", ["Image"], "Number"],
"image-pinhole-y": ["arrow", ["Image"], "Number"],
"vertical-symmetry": ["arrow", ["Image"], "Number"],
"horizontal-symmetry": ["arrow", ["Image"], "Number"],
"name-to-color": ["arrow", ["String"], "OptColor"],
"color-named": ["arrow", ["String"], "Color"],
"empty-image": "Image"
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14 changes: 14 additions & 0 deletions src/js/trove/make-image.js
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Original file line number Diff line number Diff line change
Expand Up @@ -1273,6 +1273,20 @@
return runtime.wrap(img.getHeight());
});

f("vertical-symmetry", function(maybeImg) {
checkArity(1, arguments, "vertical-symmetry", false);
c1("vertical-symmetry", maybeImg, annImage);
var img = unwrapImage(maybeImg);
return runtime.wrap(image.verticalSymmetry(img));
});

f("horizontal-symmetry", function(maybeImg) {
checkArity(1, arguments, "horizontal-symmetry", false);
c1("horizontal-symmetry", maybeImg, annImage);
var img = unwrapImage(maybeImg);
return runtime.wrap(image.horizontalSymmetry(img));
});

f("image-baseline", function(maybeImg) {
checkArity(1, arguments, "image-baseline", false);
c1("image-baseline", maybeImg, annImage);
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