Bind a dummy texture in setUniforms when compiling with emscripten

[olilarkin]

fixes the WebGL error "RENDER WARNING: there is no texture bound to the unit 0"" in Chrome

note: previous PR worked with nanovg example, but not with iPlug2, where webgl context is initially hidden. In this PR i create the dummy texture on first use which seems to work in both cases

see discussion https://github.com/memononen/nanovg/issues/398#issuecomment-437004233

[olilarkin]

nope, this is no good either, closing

[memononen]

Please update the PR instead of creating new one and closing each time. The method from MetalNanoVG should work just fine, i.e. create/find the texture in renderCreate and pass it in in setUniforms. What does not work for you?

[olilarkin]

sorry about that. first PR worked (no more webgl warning) with emscriptenized nanovg demo, but not with iPlug2, where the webgl context is not immediately onscreen. second PR worked with a basic iPlug2 project, but in this more complex demo https://iplug2.github.io/NANOVG/IPlugControls/ there were some new errors "[.WebGL-0x7fa3620ef200]GL ERROR :GL_INVALID_OPERATION : glDrawArrays: Source and destination textures of the draw are the same." And it looked totally wrong, which is why i closed the PR:

[memononen]

glDrawArrays: Source and destination textures of the draw are the same sounds like you ar rendering to a texture which you are using as a texture. Should not be related to this at all.

You have to have valid opengl context available when renderCreate is called, so creating the texture there should be valid too.

[olilarkin]

Not sure what I had done to mess it up, but this is now working with both the nanovg demo and my project, so i have modified and reopened this PR

[memononen]

Cool! I suggest following changes, I think it is safe to enable this on all cases.

Comment:

...
    // Some platforms does not allow to have samples to unset textures.
    // Create empty one which is bound when there's no texture specified.
    gl->dummyTex = glnvg__renderCreateTexture(&gl, NVG_TEXTURE_ALPHA, 1, 1, 0, NULL);

Change logic to handles missing image (should not happen!):

static void glnvg__setUniforms(GLNVGcontext* gl, int uniformOffset, int image)
{
    GLNVGtexture* tex = NULL;

    ...

    if (image != NULL) {
        tex = glnvg__findTexture(gl, image);
    }
    // If not image is set, use empty texture.
    if (tex == NULL) {
        tex = glnvg__findTexture(gl, gl->dummyTex);
    }
    glnvg__bindTexture(gl, tex != NULL ? tex->tex : 0);
    glnvg__checkError(gl, "tex paint tex");

[olilarkin]

I tried your changes, but the call to glnvgrenderCreateTexture() crashes with EXC_BAD_ACCESS in glnvgallocTexture() on macOS.

[memononen]

@olilarkin The only change to the glnvg__renderCreateTexture() bit was the comment. On second look, in that call it should be gl not &gl.

[olilarkin]

ahha, thats it. pushed some updates

[memononen]

Thanks for the fix!