OpenGL的C++：如何將32位png文件轉換爲16位565RGB文件

Question

我嘗試32位PNG文件轉換爲16位的文件格式，我知道如何轉換海誓山盟（如RGB565 RGBA4444）之間的16種文件格式但是我不知道如何去從32位轉換到16位。

我的主要問題是：如何查找32位png如何存儲（每個8位分配給R，B，G和A值）？

我該如何失去精度但仍保持大致相同的值？

在此先感謝

Answer 1

你會好得多使用libpng比手工實現這一點。

Answer 2

我不熟悉的32位PNG像素的確切佈局，但假設它是你可能想要做類似的事情到此等格式比較一致：

// Get the pixel from the png: 
unsigned int pngPixel = getPngPixel(); 

unsigned char r = (pngPixel & 0xFF000000) >> 24; 
unsigned char g = (pngPixel & 0x00FF0000) >> 16; 
unsigned char b = (pngPixel & 0x0000FF00) >> 8; 
unsigned char a = (pngPixel & 0x000000FF); 

// you can collapse this to one line, but for clarity... 
// masking off the least significant bits. 
unsigned short rgb565Pixel = (r & 0xF8) << 11; 
rgb565Pixel |= (g & 0xFC) << 5; 
rgb565Pixel |= (b & 0xF8); 

// Again you could collapse this down to one line, but for clarity... 
// masking off the least significant bits. 
unsigned short rgba4Pixel = (r & 0xF0) << 12; 
rgba4Pixel |= (g & 0xF0) << 8; 
rgba4Pixel |= (b & 0xF0) << 4; 
rgba4Pixel |= (a & 0xF0); 

考慮這個僞代碼。

有人可能會說，從8位轉換爲4位的時候屏蔽掉至少顯著位，特別是，是不是在兩者之間轉換一個很好的方法，他們會是正確的。你可以使用轉換功能：

unsigned int convertColor(unsigned char c, unsigned int oldMax, unsigned int newMax) { 
    double oldColor = c; 
    double percentOfMax = oldColor/oldMax; 
    return ((unsigned int)(newMax * percentOfMax)) & newMax; 
} 

// now we can do this 
unsigned short rgba4Pixel = convertColor(r, 0xFF, 0x0F) << 12; 
rgba4Pixel |= convertColor(g, 0xFF, 0x0F) << 8; 
rgba4Pixel |= convertColor(b, 0xFF, 0x0F) << 4; 
rgba4Pixel |= convertColor(a, 0xFF, 0x0F);