Crop image data using Renderscript Launch Options

Question

I have an byte[] array of a greyvalue image dataIn with dimension width * height. Now, I want to crop the image by applying a translation (dx, dy) and cut-off the out-of-border regions, so that the dataOut has dimension (width-abs(dx))*(height-abs(dy)).

In RenderScript I would use a 2-d uchar-Allocation both for the input and the output. In order to efficiently apply the crop operation, it was thinking of using the LaunchOptions with (for example) setX(dx,width) and setY(0, height-dy) and apply a trivial kernel that just takes values from a subset of the original dimensions.

However, when using the Launch Options, the out-Allocation still has the original size width*height, i.e. the cropped parts will just be shown as zeros - but, I actually want them to be removed, i.e. out-Allocation be of reduced dimension.

Question: is there a solution in RS in order to perform this cropping job more elegantly? Thanks for your feedback.

UPDATE: I think, I found the solution. It is by defining the out-Allocation as a script global at the reduced dimensions from the outset, passing dx and dy as well as globals and then apply rsSetElementAt_uchar to set the values of the out-Allocation. Will give an udpate later.

Answer 1

So, here is my quick rs-crop tool, taking 5ms for cropping a 500k pixel image. It uses LaunchOptions and reduced dimensions for the cropped output. Should you need to crop a Bitmap, just use element type U8_4 and allocation uchar_4 instead of U8 and uchar, respectively.

The crop.rs file:

#pragma version(1)
#pragma rs java_package_name(com.xxx.yyy)
#pragma rs_fp_relaxed

int32_t width;
int32_t height;

rs_allocation croppedImg;
uint xStart, yStart;

void __attribute__((kernel)) doCrop(uchar in,uint32_t x, uint32_t y) {
    rsSetElementAt_uchar(croppedImg,in, x-xStart, y-yStart);
}

The Java part:

// data1 is the byte[] array with (grayvalue) data of size 
// width*height you want to crop.

// define crop shift (dx, dy) here
int dx=0;       // (-width < dx < width);
int dy=250;     // (- height < dy < height);

int xStart=0, xEnd=0;
int yStart=0, yEnd=0;

// x direction
if (dx<0) {
    xStart= Math.abs(dx);
    xEnd=width;
}
else {
    xStart = 0;
    xEnd = width - Math.abs(dx);
}

// same for y direction
if (dy<0) {
    yStart= Math.abs(dy);
    yEnd=height;
}
else {
    yStart = 0;
    yEnd = height - Math.abs(dy);
}

// initiate rs and crop script
RenderScript rs = RenderScript.create(this);
ScriptC_crop mcropScr=new ScriptC_crop (rs);

// define allocations. Note the reduced size of cropAlloc 
Type.Builder typeUCHAR = new Type.Builder(rs, Element.U8(rs));  
typeUCHAR.setX(width).setY(height);

inAlloc = Allocation.createTyped(rs, typeUCHAR.create());
inAlloc.copyFrom(data1);

Type.Builder TypeUCHARCropped = new Type.Builder(rs, Element.U8(rs));
TypeUCHARCropped.setX(xEnd-xStart).setY(yEnd-yStart);

Allocation cropAlloc = Allocation.createTyped(rs,  TypeUCHARCropped.create());
mcropScr.set_croppedImg(cropAlloc);
mcropScr.set_xStart(xStart);
mcropScr.set_yStart(yStart);

Script.LaunchOptions lo = new Script.LaunchOptions();
lo.setX(xStart, xEnd);
lo.setY(yStart, yEnd);

mcropScr.forEach_doCrop(inAlloc, lo);


byte[] data1_cropped =new byte[(xEnd-xStart)*(yEnd-yStart)];
cropAlloc.copyTo(data1_cropped);