Paint Color for Image Pattern in nvgFillPaint

[xeekworx]

I needed to modulate the color of an image to be a color of my choice and couldn't find a way to do it with nanovg. For example I have an image of white text that when rendered I want to be any color I choose (eg. green). I would have though setting nvgFillColor would have done the job, but it changes my paint to a solid fill without the image pattern. So then after reading a lot of code I modified the innerColor field of my NVGpaint object with the color I wanted and it worked. There just isn't a way to do this with nvg calls that I'm aware of.

In summary: I would propose adding a NVGcolor parameter to nvgImagePattern or maybe nvgFillPaint so that I could achieve some color modulation. For the time being I'm modifying the innerColor field of my NVGpaint object.

[xeekworx]

The reason using nvgFillColor fails to change the color of my image pattern is because of nvg__setPaintColor

static void nvg__setPaintColor(NVGpaint* p, NVGcolor color)
{
    memset(p, 0, sizeof(*p)); // The image is no longer set after this
        // ...

[xeekworx]

I would offer a pull request but I'm unsure what the author really wants here. If I were to guess, for consistency with other parts of the api in painting, then nvgImagePattern needs an additional parameter for the color. For example the nvgXxxGradient calls have color parameters.

[xeekworx]

Adding another parameter would break older projects using nanovg (I must have been thinking in C++ with defaults). To avoid that either a new function would be added (eg. nvgImagePatternWithColor) or nvgFillColor could be made to just modify the inner color of NVGpaint.

[xeekworx]

You can see I'm doing it here by cheating a bit. I achieve color modulation by setting the paint_img.innerColor:

void renderer_nanovg::on_draw_image(
    uintptr_t source_img,
    const int32_t source_x, const int32_t source_y,
    const int32_t source_w, const int32_t source_h,
    const int32_t x, const int32_t y,
    const int32_t width, const int32_t height,
    const color& modulation,
    const rotation direction
) const
{
    if (m_nvgcontext) {
        NVGcontext * vg = reinterpret_cast<NVGcontext *>(m_nvgcontext);

        float angle = 0.f;
        int destX = x, destY = y;
        switch (direction) {
        case rotation::right90degrees:
            destX += height;
            angle = (90.f * NVG_PI) / 180; // Radians
            break;
        case rotation::left90degrees:
            destY += width;
            angle = (-90.f * NVG_PI) / 180; // Radians
            break;
        }

        nvgSave(vg);
        nvgTranslate(vg, (float)destX, (float)destY);

        float ax = (float)width / (float)source_w;
        float ay = (float)height / (float)source_h;
        int original_w, original_h;
        nvgImageSize(vg, static_cast<int>(source_img), &original_w, &original_h);
        NVGpaint paint_img = nvgImagePattern(
            vg, (float)(-source_x * ax), (float)(-source_y * ay), (float)original_w*ax, (float)original_h*ay, 0.f, static_cast<int>(source_img), 1.f
        );

        nvgRotate(vg, angle);

        nvgBeginPath(vg);
        nvgRect(vg, 0.f, 0.f, (float)width, (float)height);
        paint_img.innerColor = nvgRGBA(
            modulation.r,
            modulation.g,
            modulation.b,
            modulation.a);
        nvgFillPaint(vg, paint_img);
        nvgFill(vg);

        nvgRestore(vg);
    }
}