如何將真彩色圖轉換為各種灰度圖
轉自:http://www.cppblog.com/windcsn/archive/2006/07/27/Grayscale.html
首先來看一下彩色圖和灰度圖的特點。
在計算機中使用最多的 RGB 彩色空間,分別對應紅、綠、藍三種顏色;通過調配三個分量的比例來組成各種顏色。一般可以使用 1 、 2 、 4 、 8 、 16 、 24、 32 位來儲存這三顏色,不過現在一個分量最大是用 8 位來表示,最大值是 255 ,對於 32 位的顏色,高 8 位是用來表示通明度的。彩色圖一般指 16 位以上的圖。灰度圖有一個特殊之處就是組成顏色的三個分量相等;而一般灰度圖是 8 位以下。
在彩色電視機系統中,通常使用一種叫 YUV 的色彩空間,其中 Y 表示亮度訊號;也就是這個 YUV 空間解決了彩色電視機和黑白電視機的相容問題。
對於人眼來說,亮度訊號是最敏感的,如果將彩色影象轉換為灰度影象,僅僅需要轉換儲存亮度訊號就可以。
從 RGB 到 YUV 空間的 Y 轉換公式為:
Y = 0.299R+0.587G+0.114B
在 WINDOWS 中,表示 16 位以上的圖和以下的圖有點不同; 16 位以下的圖使用一個調色盤來表示選擇具體的顏色,調色盤的每個單元是 4 個位元組,其中一個透明度;而具體的畫素值儲存的是索引,分別是 1 、 2 、 4 、 8 位。 16 位以上的圖直接使用畫素表示顏色。
那麼如何將彩色圖轉換為灰度圖呢?
灰度圖中有調色盤,首先需要確定調色盤的具體顏色取值。我們前面提到了,灰度圖的三個分量相等。
當轉換為 8 位的時候,調色盤中有 256 個顏色,每個正好從 0 到 255 個,三個分量都相等。
當轉換為 4 位的時候,調色盤中 16 個顏色,等間隔平分 255 個顏色值,三個分量都相等。
當轉換為 2 位的時候,調色盤中 4 個顏色,等間隔平分 255 個顏色,三個分量相等。
當轉換為 1 位的時候,調色盤中兩個顏色,是 0 和 255 ,表示黑和白。
將彩色轉換為灰度時候,按照公式計算出對應的值,該值實際上是亮度的級別;亮度從 0 到 255 ;由於不同的位有不同的亮度級別,所以 Y 的具體取值如下:
Y = Y/ (1<<(8- 轉換的位數 ));
最後一點需要注意,得到 Y 值存放方式是不同的;分別用對應的位數來儲存對應的 Y 值。
這裡是程式碼片段:
計算調色盤和 Y 的值程式碼。
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下面是設定畫素值的程式碼:
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95
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96
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97
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98
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99
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100
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101
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102
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103
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104
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105
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106
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107
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108
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109
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110
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111
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112
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113
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114
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115
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116
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117
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118
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119
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120
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121
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122
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123
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124
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125
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126
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有一點需要說明的:
在計算 Y 值的時候,使用的整數除法,這是有誤差的,為了消除誤差,需要採用誤差擴散的演算法,也就是將該誤差值向其鄰近的想素點擴散,當然按照一定的比例來分配;例如:整除之後,餘數是 5 ,採用 3/2/3 的策略,就是,右邊畫素和正下面的畫素各佔 3/8 ,而右下角的畫素佔 2/8 。在這方面我發現 ACDSEE 做的很好,其影象的漸進做的很好。
原始碼下載:ImageConvert.zip
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