Enc: add a qmin / qmax range for quality factor

This is particularly useful for multi-pass search (but not only),
to prevent the search from going over or below a reasonable threshold.
E.g.: 'cwebp -qrange 50 80 ...' will prevent any unreasonable degradation.

new cwebp option: -qrange min max

Change-Id: I59f394533535fc20b6996bc0895f4301476d5eff

src/enc/config_enc.c

@@ -39,6 +39,8 @@ int WebPConfigInitInternal(WebPConfig* config,
   config->partitions = 0;
   config->segments = 4;
   config->pass = 1;
+  config->qmin = 0;
+  config->qmax = 100;
   config->show_compressed = 0;
   config->preprocessing = 0;
   config->autofilter = 0;
@@ -106,6 +108,9 @@ int WebPValidateConfig(const WebPConfig* config) {
   if (config->filter_type < 0 || config->filter_type > 1) return 0;
   if (config->autofilter < 0 || config->autofilter > 1) return 0;
   if (config->pass < 1 || config->pass > 10) return 0;
+  if (config->qmin < 0 || config->qmax > 100 || config->qmin > config->qmax) {
+    return 0;
+  }
   if (config->show_compressed < 0 || config->show_compressed > 1) return 0;
   if (config->preprocessing < 0 || config->preprocessing > 7) return 0;
   if (config->partitions < 0 || config->partitions > 3) return 0;

src/enc/frame_enc.c

@@ -31,10 +31,15 @@
 // we allow 2k of extra head-room in PARTITION0 limit.
 #define PARTITION0_SIZE_LIMIT ((VP8_MAX_PARTITION0_SIZE - 2048ULL) << 11)
 
+static float Clamp(float v, float min, float max) {
+  return (v < min) ? min : (v > max) ? max : v;
+}
+
 typedef struct {  // struct for organizing convergence in either size or PSNR
   int is_first;
   float dq;
   float q, last_q;
+  float qmin, qmax;
   double value, last_value;   // PSNR or size
   double target;
   int do_size_search;
@@ -47,7 +52,9 @@ static int InitPassStats(const VP8Encoder* const enc, PassStats* const s) {
 
   s->is_first = 1;
   s->dq = 10.f;
-  s->q = s->last_q = enc->config_->quality;
+  s->qmin = enc->config_->qmin;
+  s->qmax = enc->config_->qmax;
+  s->q = s->last_q = Clamp(enc->config_->quality, s->qmin, s->qmax);
   s->target = do_size_search ? (double)target_size
             : (target_PSNR > 0.) ? target_PSNR
             : 40.;   // default, just in case
@@ -56,10 +63,6 @@ static int InitPassStats(const VP8Encoder* const enc, PassStats* const s) {
   return do_size_search;
 }
 
-static float Clamp(float v, float min, float max) {
-  return (v < min) ? min : (v > max) ? max : v;
-}
-
 static float ComputeNextQ(PassStats* const s) {
   float dq;
   if (s->is_first) {
@@ -75,7 +78,7 @@ static float ComputeNextQ(PassStats* const s) {
   s->dq = Clamp(dq, -30.f, 30.f);
   s->last_q = s->q;
   s->last_value = s->value;
-  s->q = Clamp(s->q + s->dq, 0.f, 100.f);
+  s->q = Clamp(s->q + s->dq, s->qmin, s->qmax);
   return s->q;
 }
 
@@ -848,9 +851,10 @@ int VP8EncTokenLoop(VP8Encoder* const enc) {
     }
 
 #if (DEBUG_SEARCH > 0)
-    printf("#%2d metric:%.1lf -> %.1lf   last_q=%.2lf q=%.2lf dq=%.2lf\n",
+    printf("#%2d metric:%.1lf -> %.1lf   last_q=%.2lf q=%.2lf dq=%.2lf "
+           " range:[%.1f, %.1f]\n",
            num_pass_left, stats.last_value, stats.value,
-           stats.last_q, stats.q, stats.dq);
+           stats.last_q, stats.q, stats.dq, stats.qmin, stats.qmax);
 #endif
     if (enc->max_i4_header_bits_ > 0 && size_p0 > PARTITION0_SIZE_LIMIT) {
       ++num_pass_left;

src/webp/encode.h

@@ -20,7 +20,7 @@
 extern "C" {
 #endif
 
-#define WEBP_ENCODER_ABI_VERSION 0x020f    // MAJOR(8b) + MINOR(8b)
+#define WEBP_ENCODER_ABI_VERSION 0x0210    // MAJOR(8b) + MINOR(8b)
 
 // Note: forward declaring enumerations is not allowed in (strict) C and C++,
 // the types are left here for reference.
@@ -121,6 +121,8 @@ struct WebPConfig {
   int alpha_quality;      // Between 0 (smallest size) and 100 (lossless).
                           // Default is 100.
   int pass;               // number of entropy-analysis passes (in [1..10]).
+  int qmin;               // minimum permissible quality factor
+  int qmax;               // maximum permissible quality factor
 
   int show_compressed;    // if true, export the compressed picture back.
                           // In-loop filtering is not applied.