在Retina设备上升级图像

Question

I know images upscale by default on retina devices, but the default scaling makes the images blurry.

I was wondering if there was a way to scale it in nearest-neighbor mode, where there are no transparent pixels created, but rather each pixel multiplied by 4, so it looks like it would on a non retina device.

Example of what I'm talking about can be seen in the image below. example http://cclloyd.com/downloads/sdfsdf.png

Answer 1

CoreGraphics will not do a 2x scale like that, you need to write a bit of explicit pixel mapping logic to do something like this. The following is some code I used to do this operation, you would of course need to fill in the details as this operates on an input buffer of pixels and writes to an output buffer of pixels that is 2x larger.

  // Use special case "DOUBLE" logic that will simply duplicate the exact
  // RGB value from the indicated pixel into the 2x sized output buffer.

  int numOutputPixels = resizedFrameBuffer.width * resizedFrameBuffer.height;

  uint32_t *inPixels32 = (uint32_t*)cgFrameBuffer.pixels;
  uint32_t *outPixels32 = (uint32_t*)resizedFrameBuffer.pixels;

  int outRow = 0;
  int outColumn = 0;

  for (int i=0; i < numOutputPixels; i++) {
    if ((i > 0) && ((i % resizedFrameBuffer.width) == 0)) {
      outRow += 1;
      outColumn = 0;
    }

    // Divide by 2 to get the column/row in the input framebuffer
    int inColumn = outColumn / 2;
    int inRow = outRow / 2;

    // Get the pixel for the row and column this output pixel corresponds to   
    int inOffset = (inRow * cgFrameBuffer.width) + inColumn;
    uint32_t pixel = inPixels32[inOffset];

    outPixels32[i] = pixel;

    //fprintf(stdout, "Wrote 0x%.10X for 2x row/col %d %d (%d), read from row/col %d %d (%d)\n", pixel, outRow, outColumn, i, inRow, inColumn, inOffset);

    outColumn += 1;
  }

This code of course depends on you creating a buffer of pixels and then wrapping it back up into CFImageRef. But, you can find all the code to do that kind of thing easily.