5.24. Example - Using Truecolor To Make a Histogram
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## 5.24. Example - Using Truecolor To Make a Histogram

This example creates a Truecolor plot containing a histogram of a photograph, then overlays the histogram on a scaled-down copy of the photograph. The histogram is partly transparent so you can still see the photograph below. Refer to Section 4.2, “Truecolor Images” for more information on using truecolor PHPlot images.

Here are some notes on the code example:

- The main functions are `get_histogram` and `plot_histogram`. Parameters controlling the histogram and its placement on the image are in an array passed to `plot_histogram`. For the purpose of this demo, the array `$param` is used, and there is no provision to change the parameters or the photograph filename.

- This isn't a 'true' histogram, because the Y values are automatically scaled by PHPlot so they fill the available height. This could be called a 'relative histogram', with the heights indicating the relative count of pixels in the image with that value.

- The histogram is created by converting each pixel's R, G, B color values to a grayscale value between 0 and 255, and counting the number of times each value appears in the image.

- The photograph image is scaled by PHPlot to fit into the background of the plot image using SetBgImage. The histogram is then drawn into an area restricted using SetPlotAreaPixels, leaving most of the background image unobscured. All labels and tick marks are turned off. The plot data colors are set to be partly transparent using the default alpha argument to SetDataColors.

### Note

This is a demonstration only. Processing individual pixels in nested loops this way using PHP code is not recommended, because it is very slow. A small image file (800x600) might be processed in a few seconds, but a larger file such as a 12 megapixel photograph could take 30 seconds, for example.

Example 5.24. Truecolor Plot of Histogram

    <?php
    # PHPlot Example - Histogram of a Photograph
    # Display a photo image with its value histogram overlaid
    # Note: This requires PHPlot-5.1.1 or higher for Truecolor support.
    # Unlike the other examples, and contrary to the usual PHPlot recommendation,
    # this scripts creates JPEG not PNG, because most of the image is the original
    # photograph and PNG results in an overlarge file.
    require_once 'phplot.php';

    # Tunable parameters:
    $param = array(
        'plot_image_width' => 640,      # Width of final image
        'plot_image_height' => 480,     # Height of final image
        'histogram_color' => 'magenta', # Color to use for histogram lines
        'histogram_alpha' => 50,        # Histogram transparency (0=opaque, 127=clear)
        'draw_border' => True,          # If true, put a border around the histogram
        'border_color' => 'red',        # Border color, if draw_border is true
        'hx' => 0.6,                    # Upper left X relative position of histogram
        'hy' => 0.0,                    # Upper left Y relative position of histogram
        'h_width' => 0.4,               # Relative width of histogram
        'h_height' => 0.35,             # Relative height of histogram
    );

    /*
      Make a histogram from an image file, which can be palette or truecolor.
      Returns an array $histogram[i] where i is from 0 to 255. Each histogram[i]
      is the number of pixels in the image with grayscale value i.
      (Grayscale is computed using the NTSC formula, but with integers.)
    */
    function get_histogram($image_file)
    {
        list($width, $height, $imtype) = getimagesize($image_file);
        if (!empty($width)) {
            switch ($imtype) {
                case IMAGETYPE_JPEG:
                    $im = imagecreatefromjpeg($image_file);
                    break;
                case IMAGETYPE_PNG:
                    $im = imagecreatefrompng($image_file);
                    break;
                case IMAGETYPE_GIF:
                    $im = imagecreatefromgif($image_file);
                    break;
            }
        }
        if (empty($width) || empty($im)) {
            fwrite(STDERR, "Error invalid image file name: $image_file\n");
            return NULL;
        }

        # Initialize the histogram counters:
        $histogram = array_fill(0, 256, 0);

        # Process every pixel. Get the color components and compute the gray value.
        for ($y = 0; $y < $height; $y++) {
            for ($x = 0; $x < $width; $x++) {
                $pix = imagecolorsforindex($im, imagecolorat($im, $x, $y));
                $value = (int)((30 * $pix['red'] + 59 * $pix['green']
                              + 11 * $pix['blue']) / 100);
                $histogram[$value]++;
            }
        }
        return $histogram;
    }

    /*
      Make a 'plot', containing a scaled-down version of an image with
      a histogram overlay.
    */
    function plot_histogram($image_filename, $param)
    {
        extract($param);
        $histo = get_histogram($image_filename);
        if (empty($histo)) return;
        for ($i = 0; $i < 256; $i++) $data[$i] = array('', $histo[$i]);
        $p = new PHPlot_truecolor($plot_image_width, $plot_image_height);
        $p->SetFileFormat('jpg');
        $p->SetBgImage($image_filename, 'scale');
        $p->SetDataType('text-data');
        $p->SetDrawXAxis(False);
        $p->SetDrawYAxis(False);
        $p->SetDataValues($data);
        $p->SetXDataLabelPos('none');
        $p->SetXTickLabelPos('none');
        $p->SetYTickLabelPos('none');
        $p->SetXTickPos('none');
        $p->SetYTickPos('none');
        $p->SetDrawYGrid(False);
        $p->SetDataColors($histogram_color, NULL, $histogram_alpha);
        $p->SetPlotType('thinbarline');
        if ($draw_border) {
            $p->SetGridColor($border_color);
            $p->SetPlotBorderType('full');
        }
        # Compute the position of the histogram plot within the image.
        $hx0 = (int)($hx * $plot_image_width);
        $hy0 = (int)($hy * $plot_image_height);
        $hx1 = (int)($h_width * $plot_image_width) + $hx0;
        $hy1 = (int)($h_height * $plot_image_height) + $hy0;
        $p->SetPlotAreaPixels($hx0, $hy0, $hx1, $hy1);
        $p->DrawGraph();
    }

    /* Demo main. */
    plot_histogram('examples/geese.jpg', $param);

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