diff --git a/libavfilter/Makefile b/libavfilter/Makefile
index 68a94be..3231f08 100644
--- a/libavfilter/Makefile
+++ b/libavfilter/Makefile
@@ -257,10 +257,10 @@ OBJS-$(CONFIG_REPEATFIELDS_FILTER)           += vf_repeatfields.o
 OBJS-$(CONFIG_REVERSE_FILTER)                += f_reverse.o
 OBJS-$(CONFIG_ROTATE_FILTER)                 += vf_rotate.o
 OBJS-$(CONFIG_SAB_FILTER)                    += vf_sab.o
-OBJS-$(CONFIG_SCALE_FILTER)                  += vf_scale.o
-OBJS-$(CONFIG_SCALE_NPP_FILTER)              += vf_scale_npp.o
-OBJS-$(CONFIG_SCALE_VAAPI_FILTER)            += vf_scale_vaapi.o
-OBJS-$(CONFIG_SCALE2REF_FILTER)              += vf_scale.o
+OBJS-$(CONFIG_SCALE_FILTER)                  += vf_scale.o scale.o
+OBJS-$(CONFIG_SCALE_NPP_FILTER)              += vf_scale_npp.o scale.o
+OBJS-$(CONFIG_SCALE_VAAPI_FILTER)            += vf_scale_vaapi.o scale.o
+OBJS-$(CONFIG_SCALE2REF_FILTER)              += vf_scale.o scale.o
 OBJS-$(CONFIG_SELECT_FILTER)                 += f_select.o
 OBJS-$(CONFIG_SELECTIVECOLOR_FILTER)         += vf_selectivecolor.o
 OBJS-$(CONFIG_SENDCMD_FILTER)                += f_sendcmd.o
diff --git a/libavfilter/scale.c b/libavfilter/scale.c
new file mode 100644
index 0000000..b0f4be2
--- /dev/null
+++ b/libavfilter/scale.c
@@ -0,0 +1,143 @@
+/*
+ * This file is part of FFmpeg.
+ *
+ * FFmpeg is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * FFmpeg is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with FFmpeg; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+ */
+
+#include "scale.h"
+
+static const char *const var_names[] = {
+    "PI",
+    "PHI",
+    "E",
+    "in_w",   "iw",
+    "in_h",   "ih",
+    "out_w",  "ow",
+    "out_h",  "oh",
+    "a",
+    "sar",
+    "dar",
+    "hsub",
+    "vsub",
+    "ohsub",
+    "ovsub",
+    NULL
+};
+
+enum var_name {
+    VAR_PI,
+    VAR_PHI,
+    VAR_E,
+    VAR_IN_W,   VAR_IW,
+    VAR_IN_H,   VAR_IH,
+    VAR_OUT_W,  VAR_OW,
+    VAR_OUT_H,  VAR_OH,
+    VAR_A,
+    VAR_SAR,
+    VAR_DAR,
+    VAR_HSUB,
+    VAR_VSUB,
+    VAR_OHSUB,
+    VAR_OVSUB,
+    VARS_NB
+};
+
+int ff_scale_eval_dimensions(void *ctx,
+    const char *w_expr, const char *h_expr,
+    AVFilterLink *inlink, AVFilterLink *outlink,
+    int *ret_w, int *ret_h)
+{
+    const AVPixFmtDescriptor *desc = av_pix_fmt_desc_get(inlink->format);
+    const AVPixFmtDescriptor *out_desc = av_pix_fmt_desc_get(outlink->format);
+    const char *expr;
+    int w, h;
+    int factor_w, factor_h;
+    int eval_w, eval_h;
+    int ret;
+    double var_values[VARS_NB], res;
+
+    var_values[VAR_IN_W]  = var_values[VAR_IW] = inlink->w;
+    var_values[VAR_IN_H]  = var_values[VAR_IH] = inlink->h;
+    var_values[VAR_OUT_W] = var_values[VAR_OW] = NAN;
+    var_values[VAR_OUT_H] = var_values[VAR_OH] = NAN;
+    var_values[VAR_A]     = (double) inlink->w / inlink->h;
+    var_values[VAR_SAR]   = inlink->sample_aspect_ratio.num ?
+        (double) inlink->sample_aspect_ratio.num / inlink->sample_aspect_ratio.den : 1;
+    var_values[VAR_DAR]   = var_values[VAR_A] * var_values[VAR_SAR];
+    var_values[VAR_HSUB]  = 1 << desc->log2_chroma_w;
+    var_values[VAR_VSUB]  = 1 << desc->log2_chroma_h;
+    var_values[VAR_OHSUB] = 1 << out_desc->log2_chroma_w;
+    var_values[VAR_OVSUB] = 1 << out_desc->log2_chroma_h;
+
+    /* evaluate width and height */
+    av_expr_parse_and_eval(&res, (expr = w_expr),
+                           var_names, var_values,
+                           NULL, NULL, NULL, NULL, NULL, 0, ctx);
+    eval_w = var_values[VAR_OUT_W] = var_values[VAR_OW] = res;
+
+    if ((ret = av_expr_parse_and_eval(&res, (expr = h_expr),
+                                      var_names, var_values,
+                                      NULL, NULL, NULL, NULL, NULL, 0, ctx)) < 0)
+        goto fail;
+    eval_h = var_values[VAR_OUT_H] = var_values[VAR_OH] = res;
+    /* evaluate again the width, as it may depend on the output height */
+    if ((ret = av_expr_parse_and_eval(&res, (expr = w_expr),
+                                      var_names, var_values,
+                                      NULL, NULL, NULL, NULL, NULL, 0, ctx)) < 0)
+        goto fail;
+    eval_w = res;
+
+    w = eval_w;
+    h = eval_h;
+
+    /* Check if it is requested that the result has to be divisible by a some
+     * factor (w or h = -n with n being the factor). */
+    factor_w = 1;
+    factor_h = 1;
+    if (w < -1) {
+        factor_w = -w;
+    }
+    if (h < -1) {
+        factor_h = -h;
+    }
+
+    if (w < 0 && h < 0)
+        eval_w = eval_h = 0;
+
+    if (!(w = eval_w))
+        w = inlink->w;
+    if (!(h = eval_h))
+        h = inlink->h;
+
+    /* Make sure that the result is divisible by the factor we determined
+     * earlier. If no factor was set, it is nothing will happen as the default
+     * factor is 1 */
+    if (w < 0)
+        w = av_rescale(h, inlink->w, inlink->h * factor_w) * factor_w;
+    if (h < 0)
+        h = av_rescale(w, inlink->h, inlink->w * factor_h) * factor_h;
+
+    *ret_w = w;
+    *ret_h = h;
+
+    return 0;
+
+fail:
+    av_log(NULL, AV_LOG_ERROR,
+           "Error when evaluating the expression '%s'.\n"
+           "Maybe the expression for out_w:'%s' or for out_h:'%s' is self-referencing.\n",
+           expr, w_expr, h_expr);
+    return ret;
+}
diff --git a/libavfilter/scale.h b/libavfilter/scale.h
new file mode 100644
index 0000000..c95515d
--- /dev/null
+++ b/libavfilter/scale.h
@@ -0,0 +1,31 @@
+/*
+ * This file is part of FFmpeg.
+ *
+ * FFmpeg is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * FFmpeg is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with FFmpeg; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+ */
+
+#ifndef AVFILTER_SCALE_H
+#define AVFILTER_SCALE_H
+
+#include <stdint.h>
+#include "avfilter.h"
+#include "libavutil/eval.h"
+#include "libavutil/pixdesc.h"
+
+int ff_scale_eval_dimensions(void *ctx,
+    const char *w_expr, const char *h_expr,
+    AVFilterLink *inlink, AVFilterLink *outlink,
+    int *ret_w, int *ret_h);
+#endif
diff --git a/libavfilter/vf_scale.c b/libavfilter/vf_scale.c
index 22bee96..8645da7 100644
--- a/libavfilter/vf_scale.c
+++ b/libavfilter/vf_scale.c
@@ -29,9 +29,9 @@
 #include "avfilter.h"
 #include "formats.h"
 #include "internal.h"
+#include "scale.h"
 #include "video.h"
 #include "libavutil/avstring.h"
-#include "libavutil/eval.h"
 #include "libavutil/internal.h"
 #include "libavutil/mathematics.h"
 #include "libavutil/opt.h"
@@ -41,43 +41,12 @@
 #include "libavutil/avassert.h"
 #include "libswscale/swscale.h"
 
-static const char *const var_names[] = {
-    "in_w",   "iw",
-    "in_h",   "ih",
-    "out_w",  "ow",
-    "out_h",  "oh",
-    "a",
-    "sar",
-    "dar",
-    "hsub",
-    "vsub",
-    "ohsub",
-    "ovsub",
-    NULL
-};
-
-enum var_name {
-    VAR_IN_W,   VAR_IW,
-    VAR_IN_H,   VAR_IH,
-    VAR_OUT_W,  VAR_OW,
-    VAR_OUT_H,  VAR_OH,
-    VAR_A,
-    VAR_SAR,
-    VAR_DAR,
-    VAR_HSUB,
-    VAR_VSUB,
-    VAR_OHSUB,
-    VAR_OVSUB,
-    VARS_NB
-};
-
 enum EvalMode {
     EVAL_MODE_INIT,
     EVAL_MODE_FRAME,
     EVAL_MODE_NB
 };
 
-
 typedef struct ScaleContext {
     const AVClass *class;
     struct SwsContext *sws;     ///< software scaler context
@@ -256,74 +225,16 @@ static int config_props(AVFilterLink *outlink)
                             outlink->src->inputs[1] :
                             outlink->src->inputs[0];
     enum AVPixelFormat outfmt = outlink->format;
-    ScaleContext *scale = ctx->priv;
     const AVPixFmtDescriptor *desc = av_pix_fmt_desc_get(inlink->format);
-    const AVPixFmtDescriptor *out_desc = av_pix_fmt_desc_get(outlink->format);
-    int64_t w, h;
-    double var_values[VARS_NB], res;
-    char *expr;
+    ScaleContext *scale = ctx->priv;
+    int w, h;
     int ret;
-    int factor_w, factor_h;
-
-    var_values[VAR_IN_W]  = var_values[VAR_IW] = inlink->w;
-    var_values[VAR_IN_H]  = var_values[VAR_IH] = inlink->h;
-    var_values[VAR_OUT_W] = var_values[VAR_OW] = NAN;
-    var_values[VAR_OUT_H] = var_values[VAR_OH] = NAN;
-    var_values[VAR_A]     = (double) inlink->w / inlink->h;
-    var_values[VAR_SAR]   = inlink->sample_aspect_ratio.num ?
-        (double) inlink->sample_aspect_ratio.num / inlink->sample_aspect_ratio.den : 1;
-    var_values[VAR_DAR]   = var_values[VAR_A] * var_values[VAR_SAR];
-    var_values[VAR_HSUB]  = 1 << desc->log2_chroma_w;
-    var_values[VAR_VSUB]  = 1 << desc->log2_chroma_h;
-    var_values[VAR_OHSUB] = 1 << out_desc->log2_chroma_w;
-    var_values[VAR_OVSUB] = 1 << out_desc->log2_chroma_h;
-
-    /* evaluate width and height */
-    av_expr_parse_and_eval(&res, (expr = scale->w_expr),
-                           var_names, var_values,
-                           NULL, NULL, NULL, NULL, NULL, 0, ctx);
-    scale->w = var_values[VAR_OUT_W] = var_values[VAR_OW] = res;
-    if ((ret = av_expr_parse_and_eval(&res, (expr = scale->h_expr),
-                                      var_names, var_values,
-                                      NULL, NULL, NULL, NULL, NULL, 0, ctx)) < 0)
-        goto fail;
-    scale->h = var_values[VAR_OUT_H] = var_values[VAR_OH] = res;
-    /* evaluate again the width, as it may depend on the output height */
-    if ((ret = av_expr_parse_and_eval(&res, (expr = scale->w_expr),
-                                      var_names, var_values,
-                                      NULL, NULL, NULL, NULL, NULL, 0, ctx)) < 0)
-        goto fail;
-    scale->w = res;
-
-    w = scale->w;
-    h = scale->h;
-
-    /* Check if it is requested that the result has to be divisible by a some
-     * factor (w or h = -n with n being the factor). */
-    factor_w = 1;
-    factor_h = 1;
-    if (w < -1) {
-        factor_w = -w;
-    }
-    if (h < -1) {
-        factor_h = -h;
-    }
-
-    if (w < 0 && h < 0)
-        scale->w = scale->h = 0;
 
-    if (!(w = scale->w))
-        w = inlink->w;
-    if (!(h = scale->h))
-        h = inlink->h;
-
-    /* Make sure that the result is divisible by the factor we determined
-     * earlier. If no factor was set, it is nothing will happen as the default
-     * factor is 1 */
-    if (w < 0)
-        w = av_rescale(h, inlink->w, inlink->h * factor_w) * factor_w;
-    if (h < 0)
-        h = av_rescale(w, inlink->h, inlink->w * factor_h) * factor_h;
+    if ((ret = ff_scale_eval_dimensions(ctx,
+                                        scale->w_expr, scale->h_expr,
+                                        inlink, outlink,
+                                        &w, &h)) < 0)
+        goto fail;
 
     /* Note that force_original_aspect_ratio may overwrite the previous set
      * dimensions so that it is not divisible by the set factors anymore. */
@@ -439,10 +350,6 @@ static int config_props(AVFilterLink *outlink)
     return 0;
 
 fail:
-    av_log(NULL, AV_LOG_ERROR,
-           "Error when evaluating the expression '%s'.\n"
-           "Maybe the expression for out_w:'%s' or for out_h:'%s' is self-referencing.\n",
-           expr, scale->w_expr, scale->h_expr);
     return ret;
 }
 
diff --git a/libavfilter/vf_scale_npp.c b/libavfilter/vf_scale_npp.c
index 1abfdd1..84d88eb 100644
--- a/libavfilter/vf_scale_npp.c
+++ b/libavfilter/vf_scale_npp.c
@@ -27,7 +27,6 @@
 
 #include "libavutil/avstring.h"
 #include "libavutil/common.h"
-#include "libavutil/eval.h"
 #include "libavutil/hwcontext.h"
 #include "libavutil/hwcontext_cuda_internal.h"
 #include "libavutil/internal.h"
@@ -38,6 +37,7 @@
 #include "avfilter.h"
 #include "formats.h"
 #include "internal.h"
+#include "scale.h"
 #include "video.h"
 
 static const enum AVPixelFormat supported_formats[] = {
@@ -50,32 +50,6 @@ static const enum AVPixelFormat deinterleaved_formats[][2] = {
     { AV_PIX_FMT_NV12, AV_PIX_FMT_YUV420P },
 };
 
-static const char *const var_names[] = {
-    "PI",
-    "PHI",
-    "E",
-    "in_w",   "iw",
-    "in_h",   "ih",
-    "out_w",  "ow",
-    "out_h",  "oh",
-    "a", "dar",
-    "sar",
-    NULL
-};
-
-enum var_name {
-    VAR_PI,
-    VAR_PHI,
-    VAR_E,
-    VAR_IN_W,   VAR_IW,
-    VAR_IN_H,   VAR_IH,
-    VAR_OUT_W,  VAR_OW,
-    VAR_OUT_H,  VAR_OH,
-    VAR_A, VAR_DAR,
-    VAR_SAR,
-    VARS_NB
-};
-
 enum ScaleStage {
     STAGE_DEINTERLEAVE,
     STAGE_RESIZE,
@@ -359,60 +333,15 @@ static int nppscale_config_props(AVFilterLink *outlink)
 {
     AVFilterContext *ctx = outlink->src;
     AVFilterLink *inlink = outlink->src->inputs[0];
-    NPPScaleContext  *s = ctx->priv;
-    int64_t w, h;
-    double var_values[VARS_NB], res;
-    char *expr;
+    NPPScaleContext *s = ctx->priv;
+    int w, h;
     int ret;
 
-    var_values[VAR_PI]    = M_PI;
-    var_values[VAR_PHI]   = M_PHI;
-    var_values[VAR_E]     = M_E;
-    var_values[VAR_IN_W]  = var_values[VAR_IW] = inlink->w;
-    var_values[VAR_IN_H]  = var_values[VAR_IH] = inlink->h;
-    var_values[VAR_OUT_W] = var_values[VAR_OW] = NAN;
-    var_values[VAR_OUT_H] = var_values[VAR_OH] = NAN;
-    var_values[VAR_A]     = (double) inlink->w / inlink->h;
-    var_values[VAR_SAR]   = inlink->sample_aspect_ratio.num ?
-        (double) inlink->sample_aspect_ratio.num / inlink->sample_aspect_ratio.den : 1;
-    var_values[VAR_DAR]   = var_values[VAR_A] * var_values[VAR_SAR];
-
-    /* evaluate width and height */
-    av_expr_parse_and_eval(&res, (expr = s->w_expr),
-                           var_names, var_values,
-                           NULL, NULL, NULL, NULL, NULL, 0, ctx);
-    s->w = var_values[VAR_OUT_W] = var_values[VAR_OW] = res;
-    if ((ret = av_expr_parse_and_eval(&res, (expr = s->h_expr),
-                                      var_names, var_values,
-                                      NULL, NULL, NULL, NULL, NULL, 0, ctx)) < 0)
+    if ((ret = ff_scale_eval_dimensions(s,
+                                        s->w_expr, s->h_expr,
+                                        inlink, outlink,
+                                        &w, &h)) < 0)
         goto fail;
-    s->h = var_values[VAR_OUT_H] = var_values[VAR_OH] = res;
-    /* evaluate again the width, as it may depend on the output height */
-    if ((ret = av_expr_parse_and_eval(&res, (expr = s->w_expr),
-                                      var_names, var_values,
-                                      NULL, NULL, NULL, NULL, NULL, 0, ctx)) < 0)
-        goto fail;
-    s->w = res;
-
-    w = s->w;
-    h = s->h;
-
-    /* sanity check params */
-    if (w <  -1 || h <  -1) {
-        av_log(ctx, AV_LOG_ERROR, "Size values less than -1 are not acceptable.\n");
-        return AVERROR(EINVAL);
-    }
-    if (w == -1 && h == -1)
-        s->w = s->h = 0;
-
-    if (!(w = s->w))
-        w = inlink->w;
-    if (!(h = s->h))
-        h = inlink->h;
-    if (w == -1)
-        w = av_rescale(h, inlink->w, inlink->h);
-    if (h == -1)
-        h = av_rescale(w, inlink->h, inlink->w);
 
     if (w > INT_MAX || h > INT_MAX ||
         (h * inlink->w) > INT_MAX  ||
@@ -439,8 +368,6 @@ static int nppscale_config_props(AVFilterLink *outlink)
     return 0;
 
 fail:
-    av_log(NULL, AV_LOG_ERROR,
-           "Error when evaluating the expression '%s'\n", expr);
     return ret;
 }
 
diff --git a/libavfilter/vf_scale_vaapi.c b/libavfilter/vf_scale_vaapi.c
index d1cb246..1abb42e 100644
--- a/libavfilter/vf_scale_vaapi.c
+++ b/libavfilter/vf_scale_vaapi.c
@@ -31,6 +31,7 @@
 #include "avfilter.h"
 #include "formats.h"
 #include "internal.h"
+#include "scale.h"
 
 typedef struct ScaleVAAPIContext {
     const AVClass *class;
@@ -50,9 +51,13 @@ typedef struct ScaleVAAPIContext {
 
     char *output_format_string;
     enum AVPixelFormat output_format;
+
+    char *w_expr;               ///< width  expression string
+    char *h_expr;               ///< height expression string
+
+    /* computed width/height */
     int output_width;
     int output_height;
-
 } ScaleVAAPIContext;
 
 
@@ -110,6 +115,7 @@ static int scale_vaapi_config_input(AVFilterLink *inlink)
 
 static int scale_vaapi_config_output(AVFilterLink *outlink)
 {
+    AVFilterLink *inlink = outlink->src->inputs[0];
     AVFilterContext *avctx = outlink->src;
     ScaleVAAPIContext *ctx = avctx->priv;
     AVVAAPIHWConfig *hwconfig = NULL;
@@ -162,6 +168,12 @@ static int scale_vaapi_config_output(AVFilterLink *outlink)
         }
     }
 
+    if ((err = ff_scale_eval_dimensions(ctx,
+                                        ctx->w_expr, ctx->h_expr,
+                                        inlink, outlink,
+                                        &ctx->output_width, &ctx->output_height)) < 0)
+        goto fail;
+
     if (ctx->output_width  < constraints->min_width  ||
         ctx->output_height < constraints->min_height ||
         ctx->output_width  > constraints->max_width  ||
@@ -414,9 +426,9 @@ static av_cold void scale_vaapi_uninit(AVFilterContext *avctx)
 #define FLAGS (AV_OPT_FLAG_FILTERING_PARAM|AV_OPT_FLAG_VIDEO_PARAM)
 static const AVOption scale_vaapi_options[] = {
     { "w", "Output video width",
-      OFFSET(output_width),  AV_OPT_TYPE_INT, { .i64 = 0 }, 0, INT_MAX, .flags = FLAGS },
+      OFFSET(w_expr), AV_OPT_TYPE_STRING, {.str = "iw"}, .flags = FLAGS },
     { "h", "Output video height",
-      OFFSET(output_height), AV_OPT_TYPE_INT, { .i64 = 0 }, 0, INT_MAX, .flags = FLAGS },
+      OFFSET(h_expr), AV_OPT_TYPE_STRING, {.str = "ih"}, .flags = FLAGS },
     { "format", "Output video format (software format of hardware frames)",
       OFFSET(output_format_string), AV_OPT_TYPE_STRING, .flags = FLAGS },
     { NULL },
