Support writing lossless WebP

Lossless WebP is a rather interesting alternative to PNG, and already
supported by `gdImageCreateFromWebp*()`, so we add support for
`gdImageWebp*()`, too.

We can stick with the existing API, using the quality parameter to
request lossless encoding if it is set to `gdWebpLossless`, which we
define to `PHP_INT` (to avoid adding a new dependency to gd.h, we hard-
code the value – we're assuming `sizeof(int)==4` anyway).

2 files changed, 20 insertions, 1 deletions

diff --git a/src/gd.h b/src/gd.h
index 0b0cf39..3d6c0af 100644
--- a/src/gd.h
+++ b/src/gd.h
@@ -1068,6 +1068,18 @@ BGD_DECLARE(void) gdImageJpegCtx (gdImagePtr im, gdIOCtx * out, int quality);
 /* Best to free this memory with gdFree(), not free() */
 BGD_DECLARE(void *) gdImageJpegPtr (gdImagePtr im, int *size, int quality);
 
+/**
+ * Group: WebP
+ *
+ * Constant: gdWebpLossless
+ *
+ * Lossless quality
+ *
+ * See also:
+ *   - <gdImageWebpEx>
+ */
+#define gdWebpLossless 0x7FFFFFFF
+
 BGD_DECLARE(void) gdImageWebpEx (gdImagePtr im, FILE * outFile, int quantization);
 BGD_DECLARE(void) gdImageWebp (gdImagePtr im, FILE * outFile);
 BGD_DECLARE(void *) gdImageWebpPtr (gdImagePtr im, int *size);
diff --git a/src/gd_webp.c b/src/gd_webp.c
index 9886399..9b0f3fa 100644
--- a/src/gd_webp.c
+++ b/src/gd_webp.c
@@ -229,7 +229,11 @@ BGD_DECLARE(void) gdImageWebpCtx (gdImagePtr im, gdIOCtx * outfile, int quality)
 			*(p++) = a;
 		}
 	}
-	out_size = WebPEncodeRGBA(argb, gdImageSX(im), gdImageSY(im), gdImageSX(im) * 4, quality, &out);
+    if (quality == gdWebpLossless) {
+        out_size = WebPEncodeLosslessRGBA(argb, gdImageSX(im), gdImageSY(im), gdImageSX(im) * 4, &out);
+    } else {
+        out_size = WebPEncodeRGBA(argb, gdImageSX(im), gdImageSY(im), gdImageSX(im) * 4, quality, &out);
+    }
 	if (out_size == 0) {
 		gd_error("gd-webp encoding failed");
 		goto freeargb;
@@ -256,6 +260,9 @@ freeargb:
 	_quality_ should be a value in the range 0-100, higher quality values
 	usually implying both higher quality and larger image sizes.
 
+	If _quality_ is <gdWebpLossless> then the image will be written in the
+	lossless WebP format.
+
   Variants:
 
     <gdImageWebpCtx> stores the image using a <gdIOCtx> struct.