Encoding lossless AVIF is mutually exclusive with iq tuning

docs/src/content/docs/api-output.md

@@ -758,7 +758,7 @@ When using Windows ARM64, this feature requires a CPU with ARM64v8.4 or later.
 | [options.effort] | <code>number</code> | <code>4</code> | CPU effort, between 0 (fastest) and 9 (slowest) |
 | [options.chromaSubsampling] | <code>string</code> | <code>&quot;&#x27;4:4:4&#x27;&quot;</code> | set to '4:2:0' to use chroma subsampling |
 | [options.bitdepth] | <code>number</code> | <code>8</code> | set bitdepth to 8, 10 or 12 bit |
-| [options.tune] | <code>string</code> | <code>&quot;&#x27;iq&#x27;&quot;</code> | tune output for a quality metric, one of 'iq' (default), 'ssim' or 'psnr' |
+| [options.tune] | <code>string</code> | <code>&quot;&#x27;iq&#x27;&quot;</code> | tune output for a quality metric, one of 'iq' (default), 'ssim' (default when lossless) or 'psnr' |
 
 **Example**  
 ```js

lib/output.js

@@ -1175,7 +1175,7 @@ function tiff (options) {
  * @param {number} [options.effort=4] - CPU effort, between 0 (fastest) and 9 (slowest)
  * @param {string} [options.chromaSubsampling='4:4:4'] - set to '4:2:0' to use chroma subsampling
  * @param {number} [options.bitdepth=8] - set bitdepth to 8, 10 or 12 bit
- * @param {string} [options.tune='iq'] - tune output for a quality metric, one of 'iq' (default), 'ssim' or 'psnr'
+ * @param {string} [options.tune='iq'] - tune output for a quality metric, one of 'iq' (default), 'ssim' (default when lossless) or 'psnr'
  * @returns {Sharp}
  * @throws {Error} Invalid options
  */
@@ -1255,7 +1255,11 @@ function heif (options) {
     }
     if (is.defined(options.tune)) {
       if (is.string(options.tune) && is.inArray(options.tune, ['iq', 'ssim', 'psnr'])) {
+        if (this.options.heifLossless && options.tune === 'iq') {
+          this.options.heifTune = 'ssim';
+        } else {
           this.options.heifTune = options.tune;
+        }
       } else {
         throw is.invalidParameterError('tune', 'one of: psnr, ssim, iq', options.tune);
       }

test/unit/avif.js

@@ -7,7 +7,7 @@ const { describe, it } = require('node:test');
 const assert = require('node:assert');
 
 const sharp = require('../../');
-const { inputAvif, inputJpg, inputGifAnimated } = require('../fixtures');
+const { inputAvif, inputJpg, inputGifAnimated, inputPng } = require('../fixtures');
 
 describe('AVIF', () => {
   it('called without options does not throw an error', () => {
@@ -74,6 +74,35 @@ describe('AVIF', () => {
     });
   });
 
+  it('can convert PNG to lossless AVIF', async () => {
+    const data = await sharp(inputPng)
+      .resize(32)
+      .avif({ lossless: true, effort: 0 })
+      .toBuffer();
+    const { size, ...metadata } = await sharp(data).metadata();
+    void size;
+    assert.deepStrictEqual(metadata, {
+      autoOrient: {
+        height: 24,
+        width: 32
+      },
+      channels: 3,
+      compression: 'av1',
+      depth: 'uchar',
+      format: 'heif',
+      hasAlpha: false,
+      hasProfile: false,
+      height: 24,
+      isProgressive: false,
+      isPalette: false,
+      bitsPerSample: 8,
+      pagePrimary: 0,
+      pages: 1,
+      space: 'srgb',
+      width: 32
+    });
+  });
+
   it('can passthrough AVIF', async () => {
     const data = await sharp(inputAvif)
       .resize(32)