Struggling with creating new colormap for heatmap

[yassinetb]

I am trying to create a new colormap for a heatmap plot and am struggling to get the desired colors. I for some reason don't understand how the ImPlot::AddColormap method functions. Some example code is given in implot_demo, but I struggle to understand how the colors are obtained on that color map as the hex color (4282515870 etc..) equivalent (when I look up hex to RGB) seem to be of different colors than the ones obtained on the implot_demo colormap. Here is the code snippet on implot_demo for convenience (line 489):

static ImPlotColormap Liars = -1;
    if (Liars == -1) {
        static const ImU32 Liars_Data[6] = { 4282515870, 4282609140, 4287357182, 4294630301, 4294945280, 4294921472 };
        Liars = ImPlot::AddColormap("Liars", Liars_Data, 6);
    }

I am now simply trying to have three colors on my colormap: black, red and green - could you help on how to use the function or help with code to implement this simple colormap?

Thank you!

[marcizhu]

These numbers represent the colors in RGBA format, not RGB. The "A" is the alpha channel, aka the opacity of the color. An alpha of 255 would make it 100% opaque and an alpha of 0 would make it totally transparent. If you only have RGB values, you could easily make your colormap using the macro IM_COL32(R, G, B, A), where R, G, B and A are the red, green, blue and alpha channels, respectively. You can set A to 255 and it should work:

// This colormap has 3 colors: the first is pure red, the second is pure green and the third is pure blue
static const ImU32 MyColormap[] = { IM_COL32(255, 0, 0, 255), IM_COL32(0, 255, 0, 255), IM_COL32(0, 0, 255, 255) };

As an additional example, the colormap you asked for (with black, red and green colors, in that order) would be something like this:

static const ImU32 SimpleColormap[3] = { IM_COL32(0, 0, 0, 255), IM_COL32(255, 0, 0, 255), IM_COL32(0, 255, 0, 255) };
Liars = ImPlot::AddColormap("Simple", SimpleColormap, 3);

Hope that helps :)