support non 4:2:0 input in FFmpeg >= 4.1

Fall back to P010LE for 10 bit depth data and NV12 otherwise.

This may possibly lead to some loss of information
on modern hardware supporting 4:2:2 and 4:4:4 chroma subsampling
(e.g. HEVC with >= IceLake and 4:2:2 or 4:4:4 input).

Related to #26

hve.c

@@ -31,6 +31,7 @@ struct hve
 };
 
 static int init_hwframes_context(struct hve* h, const struct hve_config *config);
+static int hve_pixel_format_depth( enum AVPixelFormat pix_fmt, int *depth);
 static struct hve *hve_close_and_return_null(struct hve *h, const char *msg);
 static int HVE_ERROR_MSG(const char *msg);
 
@@ -165,10 +166,25 @@ static int init_hwframes_context(struct hve* h, const struct hve_config *config)
 
 	frames_ctx = (AVHWFramesContext*)(hw_frames_ref->data);
 	frames_ctx->format = AV_PIX_FMT_VAAPI;
-	frames_ctx->sw_format = h->sw_pix_fmt;
 	frames_ctx->width = config->width;
 	frames_ctx->height = config->height;
 	frames_ctx->initial_pool_size = 20;
+	frames_ctx->sw_format = AV_PIX_FMT_NV12;
+
+	// Starting from FFmpeg 4.1, avcodec will not fall back to NV12 automatically
+	// when using non 4:2:0 software pixel format not supported by codec.
+	// Here, instead of using h->sw_pix_fmt we always fall to P010LE for 10 bit
+	// input and NV12 otherwise which may possibly lead to some loss of information
+	// on modern hardware supporting 4:2:2 and 4:4:4 chroma subsampling
+	// (e.g. HEVC with >= IceLake)
+	// See:
+	// https://github.com/bmegli/hardware-video-encoder/issues/26
+	int depth;
+	if(hve_pixel_format_depth(h->sw_pix_fmt, &depth) != HVE_OK)
+		return HVE_ERROR_MSG("failed to get pixel format depth");
+
+	if(depth == 10)
+		frames_ctx->sw_format = AV_PIX_FMT_P010LE;
 
 	if((err = av_hwframe_ctx_init(hw_frames_ref)) < 0)
 	{
@@ -185,6 +201,22 @@ static int init_hwframes_context(struct hve* h, const struct hve_config *config)
 	return err == 0 ? HVE_OK : HVE_ERROR;
 }
 
+static int hve_pixel_format_depth(enum AVPixelFormat pix_fmt, int *depth)
+{
+	const AVPixFmtDescriptor *desc = av_pix_fmt_desc_get(pix_fmt);
+	int i;
+
+	if (!desc || !desc->nb_components)
+		return HVE_ERROR;
+
+	*depth = -INT_MAX;
+
+	for (i = 0; i < desc->nb_components; i++)
+		*depth = FFMAX(desc->comp[i].depth, *depth);
+
+	return HVE_OK;
+}
+
 int hve_send_frame(struct hve *h,struct hve_frame *frame)
 {
 	AVCodecContext* avctx=h->avctx;