[WIP] Creates an overlay for https://boards.4chan.org/wg/catalog to display and scroll through all posted images above a certain resolution.

WG-Viewer.user.js

// ==UserScript==
// @name         WG-Viewer
// @namespace    http://tampermonkey.net/
// @version      0.1
// @description  view images above a certain resolution
// @author       Max Staff <max.staff@gmx.de>
// @match        https://boards.4chan.org/wg/catalog
// @grant        GM.xmlHttpRequest
// ==/UserScript==

var minWidth = 3500;
var minHeight = 2000;
var previewImageCount = 5;
var mainImageDisplaySize = 10;

var threads = [];
var images = [];
var displayImages = {};
var imagePos = 0;
var displayContainer;
var displayNextBtn;
var displayPrevBtn;
var styleString = `
div#displayContainer {
    position: fixed;
    margin: 5%;
    padding: 10px;
    top: 0;
    right: 0;
    bottom: 0;
    left: 0;
    background-color: #000000;
    display: grid;
    grid-template-columns: [left] repeat(` + previewImageCount + `, 1fr) [center] repeat(` + previewImageCount + `, 1fr) [right];
    grid-template-rows: ` + mainImageDisplaySize + `fr 1fr auto auto;
    column-gap: 10px;
    row-gap: 10px;
    transition: 0.2s;
}

img.displayMainImage {
    grid-row: 1;
    grid-column-start: left;
    grid-column-end: right;
    object-fit: cover;
    max-height: 100%;
    max-width: 100%;
    place-self: center;
    transition: 0.2s;
}

img.displayPreviewImage {
    grid-row: 2;
    object-fit: cover;
    max-height: 100%;
    max-width: 100%;
    place-self: center;
    opacity: 0;
    cursor: pointer;
    transition: 0.2s;
}

button#displayPrevBtn {
    grid-row: 3;
    grid-column-start: left;
    grid-column-end: center;
}

button#displayNextBtn {
    grid-row: 3;
    grid-column-start: center;
    grid-column-end: right;
}

button#ballernBtn {
    grid-row: 4;
    grid-column-start: left;
    grid-column-end: right;
}
`;

var previewImageSvg = "data:image/svg+xml,%3C%3Fxml version='1.0' encoding='utf-8'%3F%3E%3Csvg xmlns='http://www.w3.org/2000/svg' xmlns:xlink='http://www.w3.org/1999/xlink' style='margin: auto; display: block; shape-rendering: auto;' width='200px' height='200px' viewBox='0 0 100 100' preserveAspectRatio='xMidYMid'%3E%3Ccircle cx='50' cy='50' r='32' stroke-width='8' stroke='%23e15b64' stroke-dasharray='50.26548245743669 50.26548245743669' fill='none' stroke-linecap='round'%3E%3CanimateTransform attributeName='transform' type='rotate' dur='1s' repeatCount='indefinite' keyTimes='0;1' values='0 50 50;360 50 50'%3E%3C/animateTransform%3E%3C/circle%3E%3Ccircle cx='50' cy='50' r='23' stroke-width='8' stroke='%23f8b26a' stroke-dasharray='36.12831551628262 36.12831551628262' stroke-dashoffset='36.12831551628262' fill='none' stroke-linecap='round'%3E%3CanimateTransform attributeName='transform' type='rotate' dur='1s' repeatCount='indefinite' keyTimes='0;1' values='0 50 50;-360 50 50'%3E%3C/animateTransform%3E%3C/circle%3E%3C/svg%3E";

function handleNextThread() {
    var thread = threads.pop();
    var req = GM.xmlHttpRequest({
        method: "GET",
        url: "https://a.4cdn.org/wg/thread/" + thread + ".json",
        onload: function(response) {
            var content = JSON.parse(response.responseText);
            for (var i = 0; i < content.posts.length; ++i) {
                content.posts[i];
                if ("tim" in content.posts[i] && "ext" in content.posts[i] && "w" in content.posts[i] && "h" in content.posts[i] && content.posts[i].w > minWidth && content.posts[i].h > minHeight) {
                    images.push("https://i.4cdn.org/wg/" + content.posts[i].tim + content.posts[i].ext);
                }
            }
            updateImages();
            console.log("Thread done: " + thread);
            window.setTimeout(handleNextThread, 1000);
        }
    });
}

function buttonClicked(e) {
    var req = GM.xmlHttpRequest({
        method: "GET",
        url: "https://a.4cdn.org/wg/threads.json",
        onload: function(response) {
            var content = JSON.parse(response.responseText);
            for (var i = 0; i < content.length; ++i) {
                for (var j = 0; j < content[i].threads.length; ++j) {
                    threads.push(content[i].threads[j].no);
                }
            }
            window.setTimeout(handleNextThread, 1000);
        }
    });
}

function nextImage(e) {
    if (images.length == 0) {
        return;
    }
    ++imagePos;
    if (imagePos >= images.length) {
        imagePos = 0;
    }
    updateImages();
}

function prevImage(e) {
    if (images.length == 0) {
        return;
    }
    --imagePos;
    if (imagePos <= 0) {
        imagePos = images.length - 1;
    }
    updateImages();
}

function updateImages() {
    if (images.length == 0) {
        return;
    }
    for (var i = 0; i < previewImageCount * 2 + 1; ++i) {
        var src = images[(imagePos - previewImageCount + i + images.length) % images.length];
        if (!(src in displayImages)) {
            var img = document.createElement("img");
            img.src = src;
            img.addEventListener("click", clickPreviewImage, true);
            img.addEventListener("load", previewImageLoaded, true);
            displayImages[src] = img;
        }
        displayImages[src].className = (i == previewImageCount) ? "displayMainImage" : "displayPreviewImage";
        displayContainer.appendChild(displayImages[src]);
    }
    for (i = 0; i < displayContainer.children.length; ++i) {
        if (displayContainer.children[i].className != "displayPreviewImage") {
            continue;
        }
        src = displayContainer.children[i].src;
        if (Math.abs(imagePos - images.indexOf(src)) > previewImageCount) {
            displayContainer.removeChild(displayContainer.children[i]);
        }
    }
}

function previewImageLoaded(e) {
    e.currentTarget.style.opacity = 1;
}

function clickPreviewImage(e) {
    if (images.length == 0) {
        return;
    }
    imagePos = images.indexOf(e.currentTarget.src);
    if (imagePos < 0 || imagePos >= images.length) {
        imagePos = 0;
    }
    updateImages();
}

function handleKeyPress(e) {
    if (e.code == "ArrowRight") {
        nextImage(e);
    } else if (e.code == "ArrowLeft") {
        prevImage(e);
    } else {
        console.log(e.code);
    }
}

function load(e) {
    if (document.getElementById("ballernBtn")) {
        return;
    }

    document.addEventListener('keydown', handleKeyPress, true);

    var style = document.createElement("style");
    style.type = "text/css";
    style.innerHTML = styleString;
    document.head.appendChild(style);

    displayContainer = document.createElement("div");
    displayContainer.id = "displayContainer";
    displayPrevBtn = document.createElement("button");
    displayPrevBtn.id = "displayPrevBtn";
    displayPrevBtn.innerHTML = "&lt;";
    displayPrevBtn.addEventListener("click", prevImage, true);
    displayNextBtn = document.createElement("button");
    displayNextBtn.id = "displayNextBtn";
    displayNextBtn.innerHTML = "&gt;";
    displayNextBtn.addEventListener("click", nextImage, true);
    var ballernBtn = document.createElement("button");
    ballernBtn.id = "ballernBtn";
    ballernBtn.innerHTML = "load images";
    ballernBtn.addEventListener("click", buttonClicked, true);
    displayContainer.appendChild(displayPrevBtn);
    displayContainer.appendChild(displayNextBtn);
    displayContainer.appendChild(ballernBtn);
    document.body.appendChild(displayContainer);
}

(function() {
    'use strict';
    document.addEventListener("load", load, true);
    // Your code here...
})();