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Javascript Controlled SVG Part 2

I have already covered utility functions and GUI code, but most of the code is for calculations of color data. This post will explain how all of the code ties together.

When the user first loads the color picker the default state is loaded. All values are zero and correspondingly the color is black. RGB (0,0,0) is black and HSV (0°,0,0) is black.

All event handlers do basically the same thing. First the master color color data  is updated to reflect the changes made by the user. Next the RGB, HSV, and hex code are updated to reflect the color data. If the user is changing RGB values, those copied to the color date and used to calculate HSV and hex code. If the user is changing HSV values, those copied to the color date and used to calculate RGB and hex code.

All event handlers ultimately call the update function. This in-turn updates the handle locations, background gradients, and preview area. each is separated into its own function.

The updateHandles function uses the color data to update the locations of the sliders. Let’s go over one group in the function.

var redHandle = max - / 255 * 199;
document.getElementById("redLabel").firstChild.nodeValue = Math.round(;

The first line is responsible for calculating the top side of the red value slider handle. Yes, I know it has magic numbers. if you read my earlier posts you would know this is more like a proof of concept than a finished product. magic numbers never make it into my finished works. 255 is the maximum value for red, 199 is for the size of the slider. Looking at it now, 199 could be replaced with (max - min). The second line moves the handle to the new position. The third updates the value displayed below the slider. These three lines are repeated for every slider with different constants.

The background of each slider shows the resulting color of moving that particular slider. These gradients need updated every time the color data changes. doing so is as easy as calculating color data as if the slider is set to each end. The hue background is slightly different. It has one gradient with all the hues represented and two overlays. one overlay is completely white representing the saturation. the other is black representing the value. The opacity of each overlay is adjusted according to the current location of the corresponding slider.

The preview is easier to compute than any topic discussed so far. Using the RGB color data, the hex code is calculated. Each color is converted using the deciToHexDigits function to ensure placement of zeros. The resulting string is then shown to the user and applied as the fill of the preview area.

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