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POc++ Pad sides of existing C array
 

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  1. POc++ Pad sides of existing C array
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    <p>I have a C array, i.e. <code>&lt;type&gt;* myarray</code>, that is logically a <code>&lt;type&gt;* myarray[#][#]</code>. It is the data from an RGBA image. I want to enlarge the "image" by shifting the contents by <code>offsetx, offsety</code> and filling the extra space with zeros.</p> <pre><code>01 02 03 04 05 06 07 08 09 10 11 12 </code></pre> <p>with offset <code>1, -1</code> yields</p> <pre><code>00 01 02 03 04 00 05 06 07 08 00 09 10 11 12 00 00 00 00 00 </code></pre> <p>There is obviously an efficient way to do this, but I'm stumped. </p> <p>Edit01: I'm trying to do something along the lines of:</p> <pre><code>/** * @brief Shifts the src image by offset(x,y) into the dst_img. * @param src_img Pointer to image data with pixels of arg_type. I.e., an * RGBA8888 image would be arg_type = Uint8. * @param dst_img Pointer to output image data. **Must** be sized to fit the source image * plus the magnitude of the offsets. I.e. a 2x2 source with 1,-1 offsets requires a * 3x3 dst. * @param src_h Height of the source * @param src_w Width of the source * @param src_channels Number of channels per pixel. I.e. RGBA8888 -&gt; 4, HSV32f32f32f -&gt; 3. * @param offset_y Vert. offset * @param offset_x Horiz. offset */ template &lt;typename arg_type&gt; void shift_image(arg_type* src_img, arg_type* dst_img, Uint32 src_h, Uint32 src_w, Uint32 src_channels, int offset_y, int offset_x) { Uint32 src_pixels = (src_h * src_w * src_channels); std::valarray&lt;arg_type&gt; srcval = std::valarray&lt;arg_type&gt;(src_img, sizeof(arg_type) * src_pixels); Uint32 dst_pixels = (src_h + abs(offset_y)) * (src_w + abs(offset_x)) * src_channels; std::valarray&lt;arg_type&gt; dstval = std::valarray&lt;arg_type&gt;((arg_type)0, sizeof(arg_type) * dst_pixels); std::slice srccolslice = std::slice(0, src_w * src_channels, 1); std::slice dstcolslice; if (offset_y &lt; 0 ) { // Top Row -&gt; Top Row for (Uint32 y = 0; y &lt; src_h; y++) { // Left Col -&gt; Left Col if (offset_x &lt; 0) { dstcolslice = std::slice(0, src_w * src_channels, 1); } else { // Right Col -&gt; Right Col dstcolslice = std::slice(offset_x, (src_w + offset_x) * src_channels, 1); } dstval[y][dstcolslice] = srcval[y][srccolslice]; } } else { // Bot Row -&gt; Bot Row for (Uint32 y = 0; y &lt; src_h; y++) { // Left Col -&gt; Left Col if (offset_x &lt; 0) { dstcolslice = std::slice(0, src_w, 1); } else { // Right Col -&gt; Right Col dstcolslice = std::slice(offset_x, (src_w + offset_x) * src_channels, 1); } dstval[y + offset_y][dstcolslice] = srcval[y][srccolslice]; } } } </code></pre>
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    1. CONo magic way to do it. Just create a second, bigger 2D array, `memset` it to 0, and then copy the contents of the first 2D to the second...
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      1. POc++ Pad sides of existing C array
      1. USFiddling Bits
    2. COHave you tried the simple approach of allocating a new array, and then filling its elements with either zero, or the corresponding element from `myarray`? Why don't you try that and post if you encounter problems.
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    3. CO@NicholasM, That was what I did initially, but the nested loops and ifs seemed insane. My insufficient grasp of the pointer math prevents a more accurate description of the problem.
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      1. USjustinzane
 

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