drawBitmap() and setPixels(): what's the stride?

Question

Could please somebody explain me (ASCII is really welcome) what the stride argument stands for in Canvas.drawBitmap() and in Bitmap.setPixels()/getPixels()? I understand it's a way to skip elements in the colors array, but how?

Answer 1

Stride is number of bytes used for storing one image row.

Stride can be different from the image width.

Most of the images are 4 byte aligned.

For ex. a 24 bit (RGB) image with width of 50 pixels. The total bytes required will be 150 (3(RGB)*50). As image will be 4 byte aligned, in this case the byte required will become 152.

So you will see stride as 152, width 50 and image alignment as 4 byte.

Answer 2

In most cases the stride is the same as the width. The stride is useful if you are trying to copy/draw a sub-region of a Bitmap. For instance, if you have a 100x100 bitmap and you want to draw the 50x50 top-right corner, you can use a width of 50px and a stride of 100px.

Answer 3

I suppose the question is about Android, java, not windows! In this case, stride has nothing to do with "number of bytes used for storing one image row", that is a windows nomenclature.

Before you understand the parameter "stride", you need to know that getPixels is a function copying pixels from the source Bitmap to destination array ( which is typed int Pixels[]).

concerning copying, you need to know where is the source (to come from), and where is the destination (to come to), in function,

public void getPixels(int[] pixels, int offset, int stride, int x, int y, int width, int height) {

throw new RuntimeException("Stub!");

}

these 4 parameters control the source: int x, int y, int width, int height

these 3 parameters control the destination: int[] pixels, int offset, int stride

e.g. You have a sourceImage with width*height = 100*100Pixels, you make a destinationImage with width*height = 200*100Pixels,and You make the following codes,

sourceImage.getPixels(pixels, 0, 2*wd, 0, 0, wd, ht); // No.1 copying

sourceImage.getPixels(pixels, wd, 2*wd, 0, 0, wd, ht);// No.2 copying

destinationImage = Bitmap.createBitmap(pixels, 0, 2*wd, 2*wd, ht, Bitmap.Config.ARGB_8888); // make a big image twice the size of the original

Explanation is given as follows for No.1 copying getPixels,

1 line reading: with line width = wd, and put it into Pixels[0]~Pixels[wd-1];

2 line reading: put it into Pixels[stride+0]~Pixels[stride+wd-1];

nth line reading: put it into Pixels[(n-1)*stride]~Pixels[(n-1)*stride+wd-1].

That is pretty much of the getPixels.