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Asked: 2026-06-09T09:40:27+00:00

I am trying to do some simple image filtering using androids ndk and seem

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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;
    }
}
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1 Answer

  1. Editorial Team

    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.

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