GStreamer播放教程05——色彩平衡

目标

      亮度，对比度，色度和饱和度都是常见的视频调节参数，也是GStreamer里面设置色彩平衡的参数。本教程将展示：

      如何发现可用的色彩平衡通道

      如何改变它们

介绍

      《GStreamer基础教程05——集成GUI工具》里面已经解释了GObject接口：应用通过它们来获得特定功能，而不用去管具体的element的实现。

      playbin2实现了色彩平衡的接口（gstcolorbalance），这就可以设置色彩平衡了。如果playbin2里面的任何一个element支持了这个接口，playbin2就仅仅简单地把应用的设置传给element，否则就会在pipeline中增加一个色彩平衡的element。

      这个接口允许查询可用的色彩平衡通道（gstcolorbalancechannel），包括它们的名字和值得有效区间，然后调整当前的值。

色彩平衡例子

#include <string.h>
#include <gst/gst.h>
#include <gst/interfaces/colorbalance.h>
  
typedef struct _CustomData {
  GstElement *pipeline;
  GMainLoop *loop;
} CustomData;
  
/* Process a color balance command */
static void update_color_channel (const gchar *channel_name, gboolean increase, GstColorBalance *cb) {
  gdouble step;
  gint value;
  GstColorBalanceChannel *channel = NULL;
  const GList *channels, *l;
  
  /* Retrieve the list of channels and locate the requested one */
  channels = gst_color_balance_list_channels (cb);
  for (l = channels; l != NULL; l = l->next) {
    GstColorBalanceChannel *tmp = (GstColorBalanceChannel *)l->data;
    
    if (g_strrstr (tmp->label, channel_name)) {
      channel = tmp;
      break;
    }
  }
  if (!channel)
    return;
  
  /* Change the channel's value */
  step = 0.1 * (channel->max_value - channel->min_value);
  value = gst_color_balance_get_value (cb, channel);
  if (increase) {
    value = (gint)(value + step);
    if (value > channel->max_value)
      value = channel->max_value;
  } else {
    value = (gint)(value - step);
    if (value < channel->min_value)
      value = channel->min_value;
  }
  gst_color_balance_set_value (cb, channel, value);
}
  
/* Output the current values of all Color Balance channels */
static void print_current_values (GstElement *pipeline) {
  const GList *channels, *l;
  
  /* Output Color Balance values */
  channels = gst_color_balance_list_channels (GST_COLOR_BALANCE (pipeline));
  for (l = channels; l != NULL; l = l->next) {
    GstColorBalanceChannel *channel = (GstColorBalanceChannel *)l->data;
    gint value = gst_color_balance_get_value (GST_COLOR_BALANCE (pipeline), channel);
    g_print ("%s: %3d%% ", channel->label,
        100 * (value - channel->min_value) / (channel->max_value - channel->min_value));
  }
  g_print ("\n");
}
  
/* Process keyboard input */
static gboolean handle_keyboard (GIOChannel *source, GIOCondition cond, CustomData *data) {
  gchar *str = NULL;
  
  if (g_io_channel_read_line (source, &str, NULL, NULL, NULL) != G_IO_STATUS_NORMAL) {
    return TRUE;
  }
  
  switch (g_ascii_tolower (str[0])) {
  case 'c':
    update_color_channel ("CONTRAST", g_ascii_isupper (str[0]), GST_COLOR_BALANCE (data->pipeline));
    break;
  case 'b':
    update_color_channel ("BRIGHTNESS", g_ascii_isupper (str[0]), GST_COLOR_BALANCE (data->pipeline));
    break;
  case 'h':
    update_color_channel ("HUE", g_ascii_isupper (str[0]), GST_COLOR_BALANCE (data->pipeline));
    break;
  case 's':
    update_color_channel ("SATURATION", g_ascii_isupper (str[0]), GST_COLOR_BALANCE (data->pipeline));
    break;
  case 'q':
    g_main_loop_quit (data->loop);
    break;
  default:
    break;
  }
  
  g_free (str);
  
  print_current_values (data->pipeline);
  
  return TRUE;
}
  
int main(int argc, char *argv[]) {
  CustomData data;
  GstStateChangeReturn ret;
  GIOChannel *io_stdin;
   
  /* Initialize GStreamer */
  gst_init (&argc, &argv);
  
  /* Initialize our data structure */
  memset (&data, 0, sizeof (data));
  
  /* Print usage map */
  g_print (
    "USAGE: Choose one of the following options, then press enter:\n"
    " 'C' to increase contrast, 'c' to decrease contrast\n"
    " 'B' to increase brightness, 'b' to decrease brightness\n"
    " 'H' to increase hue, 'h' to decrease hue\n"
    " 'S' to increase saturation, 's' to decrease saturation\n"
    " 'Q' to quit\n");
  
  /* Build the pipeline */
  data.pipeline = gst_parse_launch ("playbin2 uri=http://docs.gstreamer.com/media/sintel_trailer-480p.webm", NULL);
  
  /* Add a keyboard watch so we get notified of keystrokes */
#ifdef _WIN32
  io_stdin = g_io_channel_win32_new_fd (fileno (stdin));
#else
  io_stdin = g_io_channel_unix_new (fileno (stdin));
#endif
  g_io_add_watch (io_stdin, G_IO_IN, (GIOFunc)handle_keyboard, &data);
  
  /* Start playing */
  ret = gst_element_set_state (data.pipeline, GST_STATE_PLAYING);
  if (ret == GST_STATE_CHANGE_FAILURE) {
    g_printerr ("Unable to set the pipeline to the playing state.\n");
    gst_object_unref (data.pipeline);
    return -1;
  }
  print_current_values (data.pipeline);
   
  /* Create a GLib Main Loop and set it to run */
  data.loop = g_main_loop_new (NULL, FALSE);
  g_main_loop_run (data.loop);
  
  /* Free resources */
  g_main_loop_unref (data.loop);
  g_io_channel_unref (io_stdin);
  gst_element_set_state (data.pipeline, GST_STATE_NULL);
  gst_object_unref (data.pipeline);
  return 0;
}

工作流程

      main()函数非常的简单。用一个playbin2的建立pipeline，注册一个键盘处理函数来监控按键。

/* Output the current values of all Color Balance channels */
static void print_current_values (GstElement *pipeline) {
  const GList *channels, *l;
  
  /* Output Color Balance values */
  channels = gst_color_balance_list_channels (GST_COLOR_BALANCE (pipeline));
  for (l = channels; l != NULL; l = l->next) {
    GstColorBalanceChannel *channel = (GstColorBalanceChannel *)l->data;
    gint value = gst_color_balance_get_value (GST_COLOR_BALANCE (pipeline), channel);
    g_print ("%s: %3d%% ", channel->label,
        100 * (value - channel->min_value) / (channel->max_value - channel->min_value));
  }
  g_print ("\n");
}

      这个方法展示了如何获得通道的列表并打印所有通道当前的值。这是通过gst_color_balance_list_channels()方法来实现的，它会返回一个GList结构，我们遍历这个结构即可。

      在这个列表里面的每一个element都是GstColorBalanceChannel结构，包括通道名，最小值和最大值。然后就可以在每个通道调用gst_color_balance_get_value()方法来获得当前值。

      在这个例子中，当前值常常用占最大值的百分比来显示。

/* Process a color balance command */
static void update_color_channel (const gchar *channel_name, gboolean increase, GstColorBalance *cb) {
  gdouble step;
  gint value;
  GstColorBalanceChannel *channel = NULL;
  const GList *channels, *l;
  
  /* Retrieve the list of channels and locate the requested one */
  channels = gst_color_balance_list_channels (cb);
  for (l = channels; l != NULL; l = l->next) {
    GstColorBalanceChannel *tmp = (GstColorBalanceChannel *)l->data;
    
    if (g_strrstr (tmp->label, channel_name)) {
      channel = tmp;
      break;
    }
  }
  if (!channel)
    return;

      这个方法通过指定通道名来确定通道，然后根据操作增加或者减少值。另外，通道列表的获得后是根据指定的名字来解析获得通道的。很显然，这个列表只应该解析一次，指向通道的指针需要保持在比一个字符串更高效的数据结构中。

  /* Change the channel's value */
  step = 0.1 * (channel->max_value - channel->min_value);
  value = gst_color_balance_get_value (cb, channel);
  if (increase) {
    value = (gint)(value + step);
    if (value > channel->max_value)
      value = channel->max_value;
  } else {
    value = (gint)(value - step);
    if (value < channel->min_value)
      value = channel->min_value;
  }
  gst_color_balance_set_value (cb, channel, value);
}

      然后就获得当前通道的值，修改它但确保它的值有效，使用gst_color_balance_set_value()来设置。

      其它没有什么了。运行一下这个程序实际看一下效果。