swscale/graph: only prefer unstable backends with SWS_UNSTABLE

If the user passes `-backends all` but without `-flags unstable`, then the
default/legacy backend will be picked unless it doesn't support a given
pixel format.

This allows gradually opting into the new code to handle more pixel formats
than what the legacy backend currently supports, without disturbing the
predictable output/behavior.

Signed-off-by: Niklas Haas <git@haasn.dev>

libswscale/graph.c

@@ -658,11 +658,20 @@ static int add_convert_pass(SwsGraph *graph, const SwsFormat *src,
                             const SwsFormat *dst, SwsPass *input,
                             SwsPass **output)
 {
+    SwsContext *ctx = graph->ctx;
     int ret;
 
-    ret = add_ops_convert_pass(graph, src, dst, input, output);
-    if (ret == AVERROR(ENOTSUP))
+    if (ctx->flags & SWS_UNSTABLE) {
+        /* Prefer unstable ops backend over legacy backend */
+        ret = add_ops_convert_pass(graph, src, dst, input, output);
+        if (ret == AVERROR(ENOTSUP))
+            ret = add_legacy_sws_pass(graph, src, dst, input, output);
+    } else {
+        /* Prefer legacy backend for stability reasons */
         ret = add_legacy_sws_pass(graph, src, dst, input, output);
+        if (ret == AVERROR(ENOTSUP))
+            ret = add_ops_convert_pass(graph, src, dst, input, output);
+    }
 
     return ret;
 }

libswscale/swscale.h

@@ -180,9 +180,9 @@ typedef enum SwsFlags {
     SWS_BITEXACT       = 1 << 19,
 
     /**
-     * Allow using experimental new code paths. This may be faster, slower,
-     * or produce different output, with semantics subject to change at any
-     * point in time. For testing and debugging purposes only.
+     * Allow/prefer using experimental new code paths. This may be faster,
+     * slower, or produce different output, with semantics subject to change
+     * at any point in time. For testing and debugging purposes only.
      */
     SWS_UNSTABLE = 1 << 20,