Save PNG compressed, WebP at effort 4 and AVIF at effort 1 (closes #232)

Each output format now has its own govips export with its settings
named, in place of govips' generic Export, which sent libvips a zero for
some settings it was not given: PNG had no compression and WebP effort
0. PNG now gets libvips' default compression, 6, and WebP its default
effort, 4. GIF and JPEG output is unchanged.

AVIF was at libvips' default effort, 4, which takes minutes for an
8192x8192 image with one libvips thread, far past the default
downstream_timeout. Effort 1, the lowest govips can set, takes about 51
seconds for an image of random pixels, the worst case, and WebP at 4
about 43. A 16-bit source gets 8 bits per sample, not libvips' 12, which
take over four times as long.

Model: opus-5-5

internal/imageprocessor/imageprocessor.go

@@ -504,36 +504,21 @@ func (p *ImageProcessor) encode(
 		}
 	}
 
-	var params vips.ExportParams
-
 	switch format {
 	case FormatJPEG:
-		params = vips.ExportParams{
-			Format:  vips.ImageTypeJPEG,
-			Quality: quality,
-		}
+		return exportJPEG(img, quality)
 
 	case FormatPNG:
-		params = vips.ExportParams{
-			Format: vips.ImageTypePNG,
-		}
+		return exportPNG(img)
 
 	case FormatGIF:
-		params = vips.ExportParams{
-			Format: vips.ImageTypeGIF,
-		}
+		return exportGIF(img)
 
 	case FormatWebP:
-		params = vips.ExportParams{
-			Format:  vips.ImageTypeWEBP,
-			Quality: quality,
-		}
+		return exportWebP(img, quality)
 
 	case FormatAVIF:
-		params = vips.ExportParams{
-			Format:  vips.ImageTypeAVIF,
-			Quality: quality,
-		}
+		return exportAVIF(img, quality)
 
 	case FormatJXL:
 		return exportJXL(img, quality)
@@ -544,17 +529,90 @@ func (p *ImageProcessor) encode(
 	default:
 		return nil, fmt.Errorf("%w: %s", ErrUnsupportedOutputFormat, format)
 	}
+}
 
-	// Drop EXIF, XMP, IPTC and the ICC profile. govips ignores this for
-	// GIF, which carries none of them.
-	params.StripMetadata = true
+// govips sends libvips Go's zero value for some settings an export leaves
+// out, such as no compression at all for PNG, so each export below sets
+// every setting whose zero value is not what pixa wants. Stripping metadata
+// drops EXIF, XMP, IPTC and the ICC profile.
 
-	output, _, err := img.Export(&params)
-	if err != nil {
-		return nil, err
-	}
+// exportJPEG encodes img as JPEG at quality, without metadata. The settings
+// it leaves out are at libvips' defaults.
+func exportJPEG(img *vips.ImageRef, quality int) ([]byte, error) {
+	output, _, err := img.ExportJpeg(&vips.JpegExportParams{
+		StripMetadata: true,
+		Quality:       quality,
+	})
 
-	return output, nil
+	return output, err
+}
+
+// pngCompression is libvips' default PNG compression, from 0 (none) to 9.
+const pngCompression = 6
+
+// exportPNG encodes img as PNG at libvips' default compression and row
+// filter, without metadata.
+func exportPNG(img *vips.ImageRef) ([]byte, error) {
+	output, _, err := img.ExportPng(&vips.PngExportParams{
+		StripMetadata: true,
+		Compression:   pngCompression,
+		Filter:        vips.PngFilterNone,
+	})
+
+	return output, err
+}
+
+// gifEffort is libvips' default GIF effort, from 1 to 10.
+const gifEffort = 7
+
+// exportGIF encodes img as GIF at libvips' default effort. govips cannot
+// have libvips strip metadata from GIF, which carries none.
+func exportGIF(img *vips.ImageRef) ([]byte, error) {
+	output, _, err := img.ExportGIF(&vips.GifExportParams{Effort: gifEffort})
+
+	return output, err
+}
+
+// webpEffort is libvips' default WebP effort, from 0 (fastest) to 6.
+const webpEffort = 4
+
+// exportWebP encodes img as lossy WebP at quality and libvips' default
+// effort, without metadata.
+func exportWebP(img *vips.ImageRef, quality int) ([]byte, error) {
+	output, _, err := img.ExportWebp(&vips.WebpExportParams{
+		StripMetadata:   true,
+		Quality:         quality,
+		ReductionEffort: webpEffort,
+	})
+
+	return output, err
+}
+
+// avifEffort is the AVIF effort, from 0 (fastest) to 9; 1 is the lowest
+// govips can set. With one thread, as pixad runs libvips, 1 takes about 51
+// seconds to save an 8192x8192 image of random pixels, the worst case,
+// against the default downstream_timeout of 60 seconds. On an image of
+// milder noise, which 1 saves in about 12 seconds, 2 takes nearly a minute
+// and libvips' default, 4, takes minutes.
+const avifEffort = 1
+
+// avifBitdepth is the AVIF bit depth, 8 bits per sample for every image.
+// libvips would save a 16-bit image with 12, but at avifEffort that takes
+// about 54 seconds for a 16-bit 8192x8192 image of milder noise, nearly all
+// of the default downstream_timeout, and about 12 seconds with 8.
+const avifBitdepth = 8
+
+// exportAVIF encodes img as lossy AVIF at quality, avifEffort and
+// avifBitdepth, without metadata.
+func exportAVIF(img *vips.ImageRef, quality int) ([]byte, error) {
+	output, _, err := img.ExportAvif(&vips.AvifExportParams{
+		StripMetadata: true,
+		Quality:       quality,
+		Effort:        avifEffort,
+		Bitdepth:      avifBitdepth,
+	})
+
+	return output, err
 }
 
 // jxlResolution is the resolution every JPEG XL image is saved with, in