Java image conversion to RGB565

问题:

I try to convert image to RGB565 format. I read this image:

BufferedImage bufImg = ImageIO.read(imagePathFile);
sendImg = new BufferedImage(CONTROLLER_LCD_WIDTH/*320*/, CONTROLLER_LCD_HEIGHT/*240*/, BufferedImage.TYPE_USHORT_565_RGB);
sendImg.getGraphics().drawImage(bufImg, 0, 0, CONTROLLER_LCD_WIDTH/*320*/, CONTROLLER_LCD_HEIGHT/*240*/, null);

Here is it:

Then I convert it to RGB565:

int numByte=0;
byte[] OutputImageArray = new byte[CONTROLLER_LCD_WIDTH*CONTROLLER_LCD_HEIGHT*2];

int i=0;
int j=0;
int len = OutputImageArray.length;

for (i=0;i<CONTROLLER_LCD_WIDTH;i++) {
    for (j=0;j<CONTROLLER_LCD_HEIGHT;j++) {

        Color c = new Color(sendImg.getRGB(i, j));
        int aRGBpix = sendImg.getRGB(i, j);
        int alpha;
        int red = c.getRed();
        int green = c.getGreen();
        int blue = c.getBlue();

        //RGB888
        red = (aRGBpix >> 16) & 0x0FF;
        green = (aRGBpix >> 8) & 0x0FF;
        blue = (aRGBpix >> 0) & 0x0FF; 
        alpha = (aRGBpix >> 24) & 0x0FF;

        //RGB565
        red = red >> 3;
        green = green >> 2;
        blue = blue >> 3;

        //A pixel is represented by a 4-byte (32 bit) integer, like so:
        //00000000 00000000 00000000 11111111
        //^ Alpha  ^Red     ^Green   ^Blue
        //Converting to RGB565

        short pixel_to_send = 0;
        int pixel_to_send_int = 0;
        pixel_to_send_int = (red << 11) | (green << 5) | (blue);
        pixel_to_send = (short) pixel_to_send_int;


        //dividing into bytes
        byte byteH=(byte)((pixel_to_send >> 8) & 0x0FF);
        byte byteL=(byte)(pixel_to_send & 0x0FF);

        //Writing it to array - High-byte is second
        OutputImageArray[numByte]=byteH;
        OutputImageArray[numByte+1]=byteL;

        numByte+=2;
    }
}

Then I try to restore this from resulting array OutputImageArray:

i=0;
j=0;                        
numByte=0;
BufferedImage NewImg = new BufferedImage(CONTROLLER_LCD_WIDTH, CONTROLLER_LCD_HEIGHT, BufferedImage.TYPE_USHORT_565_RGB);
for (i=0;i<CONTROLLER_LCD_WIDTH;i++) {
    for (j=0;j<CONTROLLER_LCD_HEIGHT;j++) {

        int curPixel=0;
        int alpha=0x0FF;
        int red;
        int green;
        int blue; 

        byte byteL=0;
        byte byteH=0;

        byteH = OutputImageArray[numByte];
        byteL = OutputImageArray[numByte+1];

        curPixel= (byteH << 8) | (byteL);

        //RGB565
        red = (curPixel >> (6+5)) & 0x01F;
        green = (curPixel >> 5) & 0x03F;
        blue = (curPixel) & 0x01F;

        //RGB888
        red = red << 3;
        green = green << 2;
        blue = blue << 3;                                

        //aRGB
        curPixel = 0;
        curPixel = (alpha << 24) | (red << 16) | (green << 8) | (blue);

        NewImg.setRGB(i, j, curPixel);
        numByte+=2;

    }
}

I output this restored image. But I see that it looks very poor.

I expected the lost of pictures quality. But as I thought, this picture has to have almost the same quality as the previous picture. Is it right? Is my code right?

回答1:

The reason you see these yellow artefacts is simply due to negative values for byteL overwriting bits from byteH containing the correct values for red and (part of) green channels. Let me explain.

Remember, if the highest bit in a byte is set to 1, the value is considered negative (-128 to -1 instead of 128 to 255), and by converting it to an int all the extra high-bits are set to 1 to conserve the same values (-128 to -1).

In your program, these extra bits set to 1 are in direct conflict with the value in byteH when applying the OR bit-operator |, overwriting (saturating) the red and (part of) green values you are trying to extract and display.

curPixel = (byteH << 8) | (byteL); // BUG: issue with negative byteL values

A solution is to apply an AND-mask to be sure to get rid of any unwanted bits before applying the OR bit-operator.

curPixel = byteL & 0xFF; // Convert byte to int to be within [0 , 255]
curPixel = (byteH << 8) | curPixel; // Apply OR bit-operator