readPNG函數的結果是什麼？

Question

我不認爲我理解功能readPNG。在幫助我們可以發現這一段代碼

img <- readPNG(system.file("img", "Rlogo.png", package="png")) 

img是numieric向量NUM [1:76 1：100 1：4]（但它是在四個矩陣形式dispalyed）

當我讀了另一個PNG文件（25 x 25，邏輯[只有黑色或白色像素]），我得到了矢量數[1:25 1:25 1：3]。看起來這三個邏輯矩陣是相同的。你能解釋一下readPNG函數的結果嗎？爲什麼一旦它顯示爲4個矩陣，一次是三個？

Answer 1

雖然我會承認許多R手冊頁是鈍的，這一個是相當不錯的。

Value 

If native is FALSE then an array of the dimensions height x width x 
channels. If there is only one channel the result is a matrix. The 
values are reals between 0 and 1. If native is TRUE then an object of 
the class nativeRaster is returned instead. The latter cannot be 
easily computed on but is the most efficient way to draw using 
rasterImage. 

Most common files decompress into RGB (3 channels), RGBA (4 channels), 
Grayscale (1 channel) or GA (2 channels). Note that G and GA images 
cannot be directly used in rasterImage unless native is set to TRUE 
because rasterImage requires RGB or RGBA format (nativeRaster is 
always 8-bit RGBA). 

As of png 0.1-2 files with 16-bit channels are converted in full 
resolution to the array format, but the nativeRaster format only 
supports 8-bit and therefore a truncation is performed (eight least 
significant bits are dropped) with a warning if native is TRUE.