diff --git a/src/frameLoader.worker.ts b/src/frameLoader.worker.ts
new file mode 100644
index 0000000..a3014a5
--- /dev/null
+++ b/src/frameLoader.worker.ts
@@ -0,0 +1,51 @@
+import { parentPort, workerData } from "worker_threads";
+import fs from "fs";
+
+function loadPacked(filename: string): {
+    fps: number;
+    lengths: Uint32Array;
+    packed: Uint8Array;
+} {
+    const parsed = JSON.parse(fs.readFileSync(filename, "utf8"));
+
+    if (
+        !parsed ||
+        !Array.isArray(parsed.frames) ||
+        parsed.frames.length === 0 ||
+        typeof parsed.fps !== "number" ||
+        !Number.isFinite(parsed.fps) ||
+        parsed.fps <= 0
+    ) {
+        throw new Error(
+            `Invalid frames file "${filename}": expected non-empty frames[] and a positive fps`
+        );
+    }
+
+    const raw = parsed.frames as unknown[];
+    const count = raw.length;
+    const lengths = new Uint32Array(count);
+    const bufs: Buffer[] = new Array(count);
+    let total = 0;
+    for (let i = 0; i < count; i++) {
+        const b = Buffer.from(raw[i] as Uint8Array);
+        bufs[i] = b;
+        lengths[i] = b.length;
+        total += b.length;
+    }
+
+    const packed = new Uint8Array(total);
+    let off = 0;
+    for (let i = 0; i < count; i++) {
+        packed.set(bufs[i], off);
+        off += lengths[i];
+    }
+
+    return { fps: parsed.fps, lengths, packed };
+}
+
+const { filename } = workerData as { filename: string };
+const { fps, lengths, packed } = loadPacked(filename);
+parentPort!.postMessage(
+    { fps, lengths: lengths.buffer, packed: packed.buffer },
+    [lengths.buffer, packed.buffer] as never
+);
diff --git a/src/frames.test.ts b/src/frames.test.ts
index d82e5d6..83a790b 100644
--- a/src/frames.test.ts
+++ b/src/frames.test.ts
@@ -1,31 +1,96 @@
 import { describe, test, expect } from "bun:test";
-import { frameToAscii } from "./frames";
+import { frameToAscii, resolveCharset, CHARSET_PRESETS } from "./frames";
 
 describe("frameToAscii", () => {
     test("returns empty string for an empty buffer", () => {
-        expect(frameToAscii(Buffer.from([]), 40)).toBe("");
+        expect(frameToAscii(Buffer.from([]), { brightnessThreshold: 40 })).toBe(
+            ""
+        );
     });
 
     test("maps a zero-brightness pixel to the first (blankest) character", () => {
-        expect(frameToAscii(Buffer.from([0]), 40)).toBe(" ");
+        expect(
+            frameToAscii(Buffer.from([0]), { brightnessThreshold: 40 })
+        ).toBe(" ");
     });
 
     test("maps a below-threshold pixel to the first character", () => {
         // brightness = floor((50/255)*100) = 19, below threshold 40
-        expect(frameToAscii(Buffer.from([50]), 40)).toBe(" ");
+        expect(
+            frameToAscii(Buffer.from([50]), { brightnessThreshold: 40 })
+        ).toBe(" ");
     });
 
     test("maps a mid-range pixel to a mid-range character", () => {
         // brightness = floor((128/255)*100) = 50
-        expect(frameToAscii(Buffer.from([128]), 40)).toBe("n");
+        expect(
+            frameToAscii(Buffer.from([128]), { brightnessThreshold: 40 })
+        ).toBe("n");
     });
 
     test("maps a max-brightness pixel to the last (densest) character", () => {
-        // brightness = floor((255/255)*100) = 100, must clamp to last index
-        expect(frameToAscii(Buffer.from([255]), 40)).toBe("$");
+        expect(
+            frameToAscii(Buffer.from([255]), { brightnessThreshold: 40 })
+        ).toBe("$");
     });
 
     test("maps multiple pixels in sequence, preserving order", () => {
-        expect(frameToAscii(Buffer.from([0, 128, 255]), 40)).toBe(" n$");
+        expect(
+            frameToAscii(Buffer.from([0, 128, 255]), {
+                brightnessThreshold: 40,
+            })
+        ).toBe(" n$");
+    });
+
+    test("uses a named charset preset", () => {
+        // standard ramp " .:-=+*#%@": max brightness -> last char "@"
+        expect(
+            frameToAscii(Buffer.from([255]), {
+                brightnessThreshold: 40,
+                charset: "standard",
+            })
+        ).toBe("@");
+    });
+
+    test("accepts a custom literal ramp", () => {
+        expect(
+            frameToAscii(Buffer.from([0, 255]), {
+                brightnessThreshold: 0,
+                charset: "AB",
+            })
+        ).toBe("AB");
+    });
+
+    test("invert reverses the ramp", () => {
+        expect(
+            frameToAscii(Buffer.from([255]), {
+                brightnessThreshold: 40,
+                charset: "AB",
+                invert: true,
+            })
+        ).toBe("A");
+    });
+
+    test("supports multi-byte (Unicode) ramps", () => {
+        expect(
+            frameToAscii(Buffer.from([255]), {
+                brightnessThreshold: 40,
+                charset: "blocks",
+            })
+        ).toBe("█");
+    });
+});
+
+describe("resolveCharset", () => {
+    test("returns the detailed preset by default", () => {
+        expect(resolveCharset()).toBe(CHARSET_PRESETS.detailed);
+    });
+
+    test("resolves preset names case-insensitively", () => {
+        expect(resolveCharset("BLOCKS")).toBe(CHARSET_PRESETS.blocks);
+    });
+
+    test("passes through a custom ramp", () => {
+        expect(resolveCharset(" .#@")).toBe(" .#@");
     });
 });
diff --git a/src/frames.ts b/src/frames.ts
index 9981c74..2fc5c86 100644
--- a/src/frames.ts
+++ b/src/frames.ts
@@ -1,16 +1,82 @@
 import fs from "fs";
+import path from "path";
+import { Worker } from "worker_threads";
 import sharp from "sharp";
 
+const WORKER_PATH = path.join(__dirname, "frameLoader.worker.ts");
+
 export interface FramesContainer {
     frames: Buffer[];
     fps: number;
+    name?: string;
 }
 
-const PIXEL_CHARS =
-    " .'`^\",:;Il!i><~+_-?][}{1)(|/tfjrxnuvczXYUJCLQ0OZmwqpdbkhao*#MW&8%B@$";
+export const CHARSET_PRESETS: Record<string, string> = {
+    detailed:
+        " .'`^\",:;Il!i><~+_-?][}{1)(|/tfjrxnuvczXYUJCLQ0OZmwqpdbkhao*#MW&8%B@$",
+    standard: " .:-=+*#%@",
+    simple: " .:oO#@",
+    blocks: " ░▒▓█",
+};
+
+const DEFAULT_CHARSET = CHARSET_PRESETS.detailed;
+
+export interface AsciiOptions {
+    brightnessThreshold?: number;
+    charset?: string;
+    invert?: boolean;
+}
+
+export function resolveCharset(charset?: string): string {
+    if (!charset) return DEFAULT_CHARSET;
+    return CHARSET_PRESETS[charset.toLowerCase()] ?? charset;
+}
 
 export function loadFrames(filename: string): FramesContainer {
-    return JSON.parse(fs.readFileSync(filename).toString());
+    const parsed = JSON.parse(fs.readFileSync(filename).toString());
+
+    if (
+        !parsed ||
+        !Array.isArray(parsed.frames) ||
+        parsed.frames.length === 0 ||
+        typeof parsed.fps !== "number" ||
+        !Number.isFinite(parsed.fps) ||
+        parsed.fps <= 0
+    ) {
+        throw new Error(
+            `Invalid frames file "${filename}": expected non-empty frames[] and a positive fps`
+        );
+    }
+
+    return parsed as FramesContainer;
+}
+
+export function loadFramesAsync(filename: string): Promise<FramesContainer> {
+    return new Promise((resolve, reject) => {
+        const worker = new Worker(WORKER_PATH, { workerData: { filename } });
+        worker.once(
+            "message",
+            (msg: {
+                fps: number;
+                lengths: ArrayBuffer;
+                packed: ArrayBuffer;
+            }) => {
+                const lengths = new Uint32Array(msg.lengths);
+                const frames: Buffer[] = new Array(lengths.length);
+                let off = 0;
+                for (let i = 0; i < lengths.length; i++) {
+                    frames[i] = Buffer.from(msg.packed, off, lengths[i]);
+                    off += lengths[i];
+                }
+                resolve({ frames, fps: msg.fps });
+                worker.terminate();
+            }
+        );
+        worker.once("error", (err) => {
+            worker.terminate();
+            reject(err);
+        });
+    });
 }
 
 export async function resizeFrame(
@@ -22,24 +88,65 @@ export async function resizeFrame(
     return sharp(frame)
         .resize(width, height, {
             fit: keepAspectRatio ? "contain" : "fill",
-        })
-        .grayscale()
-        .extend({
-            top: 0,
-            bottom: 0,
-            left: 0,
-            right: 0,
             background: { r: 0, g: 0, b: 0, alpha: 1 },
         })
+        .grayscale()
+        .removeAlpha()
         .raw()
         .toBuffer();
 }
 
+export class FrameRenderer {
+    private cache = new Map<string, string>();
+
+    constructor(
+        private readonly frames: Buffer[],
+        private readonly options: AsciiOptions,
+        private readonly maxEntries = 4096
+    ) {}
+
+    async render(
+        index: number,
+        width: number,
+        height: number,
+        keepAspectRatio: boolean
+    ): Promise<string> {
+        const key = `${index}:${width}x${height}:${keepAspectRatio ? 1 : 0}`;
+        const cached = this.cache.get(key);
+        if (cached !== undefined) {
+            this.cache.delete(key);
+            this.cache.set(key, cached);
+            return cached;
+        }
+
+        const source = Buffer.isBuffer(this.frames[index])
+            ? this.frames[index]
+            : Buffer.from(this.frames[index] as unknown as Uint8Array);
+        const resized = await resizeFrame(
+            source,
+            width,
+            height,
+            keepAspectRatio
+        );
+        const ascii = frameToAscii(resized, this.options);
+
+        this.cache.set(key, ascii);
+        if (this.cache.size > this.maxEntries) {
+            const oldest = this.cache.keys().next().value;
+            if (oldest !== undefined) this.cache.delete(oldest);
+        }
+        return ascii;
+    }
+}
+
 export function frameToAscii(
     pixels: Buffer,
-    brightnessThreshold: number
+    options: AsciiOptions = {}
 ): string {
-    const totalPixelColors = PIXEL_CHARS.length;
+    const { brightnessThreshold = 40, charset, invert = false } = options;
+
+    const ramp = [...resolveCharset(charset)];
+    const total = ramp.length;
     let result = "";
 
     for (let i = 0; i < pixels.length; i++) {
@@ -49,13 +156,11 @@ export function frameToAscii(
         if (brightness < brightnessThreshold) {
             index = 0;
         } else {
-            index = Math.min(
-                Math.floor((brightness / 100) * totalPixelColors),
-                totalPixelColors - 1
-            );
+            index = Math.min(Math.floor((brightness / 100) * total), total - 1);
         }
 
-        result += PIXEL_CHARS[index];
+        if (invert) index = total - 1 - index;
+        result += ramp[index];
     }
 
     return result;