CODEWITHSUNDEEP
androidcimage-processingandroid-ndkbit-manipulation
Mr Zorn
Mr Zorn

Reputation: 1913

Android NDK set RGB bitmap pixels

I am trying to do some simple image filtering using androids ndk and seem to be having some issues with getting and setting the rgb values of the bitmap.

I have stripped out all the actual processing and am just trying to set every pixel of the bitmap to red, but I end up with a blue image instead. I assume there is something simple that I have overlooked but any help is appreciated.

static void changeIt(AndroidBitmapInfo* info, void* pixels){
int x, y, red, green, blue;

for (y=0;y<info->height;y++) {


     uint32_t * line = (uint32_t *)pixels;
        for (x=0;x<info->width;x++) {

            //get the values
            red = (int) ((line[x] & 0xFF0000) >> 16);
            green = (int)((line[x] & 0x00FF00) >> 8);
            blue = (int) (line[x] & 0x0000FF);

            //just set it to all be red for testing
            red = 255;
            green = 0;
            blue = 0;

            //why is the image totally blue??
            line[x] =
              ((red << 16) & 0xFF0000) |
              ((green << 8) & 0x00FF00) |
              (blue & 0x0000FF);
        }

        pixels = (char *)pixels + info->stride;
    }
}

How should I both get and then set the rgb values for each pixel??

Update with answer
As pointed out below it seems that little endian is used, so in my original code I just had to switch the red and blue variables:

static void changeIt(AndroidBitmapInfo* info, void* pixels){
int x, y, red, green, blue;

for (y=0;y<info->height;y++) {


     uint32_t * line = (uint32_t *)pixels;
        for (x=0;x<info->width;x++) {

            //get the values
            blue = (int) ((line[x] & 0xFF0000) >> 16);
            green = (int)((line[x] & 0x00FF00) >> 8);
            red = (int) (line[x] & 0x0000FF);

            //just set it to all be red for testing
            red = 255;
            green = 0;
            blue = 0;

            //why is the image totally blue??
            line[x] =
              ((blue<< 16) & 0xFF0000) |
              ((green << 8) & 0x00FF00) |
              (red & 0x0000FF);
        }

        pixels = (char *)pixels + info->stride;
    }
}

Upvotes: 2

Views: 3997

Answers (1)

user1157391
user1157391

Reputation:

It depends on the pixel format. Presumably your bitmap is in RGBA. So 0x00FF0000 corresponds to the byte sequence 0x00, 0x00, 0xFF, 0x00 (little endian), that is blue with transparency 0.

I am not an Android developer so I don't know if there are helper functions to get/set color components or if you have to do it yourself, based on the AndroidBitmapInfo.format field. You'll have to read the API documentation.

Upvotes: 2

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