juce::OpenGLTexture::loadAlpha() Produces GL_INVALID_ENUM

General JUCE discussion
1

Hi all, I was creating an array of alpha values to use for an OpenGL texture and ran into GL_INVALID_ENUM. The error gets generated from juce::OpenGLTexture::create():

void OpenGLTexture::create (const int w, const int h, const void* pixels, GLenum type, bool topLeft)
{
    ownerContext = OpenGLContext::getCurrentContext();

    // Texture objects can only be created when the current thread has an active OpenGL
    // context. You'll need to create this object in one of the OpenGLContext's callbacks.
    jassert (ownerContext != nullptr);

    if (textureID == 0)
    {
        JUCE_CHECK_OPENGL_ERROR
        glGenTextures (1, &textureID);
        glBindTexture (GL_TEXTURE_2D, textureID);
        glTexParameteri (GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);

        auto glMagFilter = (ownerContext->texMagFilter == OpenGLContext::linear ? GL_LINEAR : GL_NEAREST);
        glTexParameteri (GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, glMagFilter);
        glTexParameteri (GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
        glTexParameteri (GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);
        JUCE_CHECK_OPENGL_ERROR
    }
    else
    {
        glBindTexture (GL_TEXTURE_2D, textureID);
        JUCE_CHECK_OPENGL_ERROR;
    }

    glPixelStorei (GL_UNPACK_ALIGNMENT, 1);
    JUCE_CHECK_OPENGL_ERROR

    width  = getAllowedTextureSize (w);
    height = getAllowedTextureSize (h);

    const GLint internalformat = type == GL_ALPHA ? GL_ALPHA : GL_RGBA;

    if (width != w || height != h)
    {
        glTexImage2D (GL_TEXTURE_2D, 0, internalformat,
                      width, height, 0, type, GL_UNSIGNED_BYTE, nullptr);

        glTexSubImage2D (GL_TEXTURE_2D, 0, 0, topLeft ? (height - h) : 0, w, h,
                         type, GL_UNSIGNED_BYTE, pixels);
    }
    else
    {
        glTexImage2D (GL_TEXTURE_2D, 0, internalformat,
                      w, h, 0, type, GL_UNSIGNED_BYTE, pixels);
    }

    JUCE_CHECK_OPENGL_ERROR // <<<<<<<<< Generated here
}

type in this context is GL_ALPHA, but GL_ALPHA for the internalFormat and type arguments seems to only be accepted in versions before OpenGL 3

https://www.khronos.org/registry/OpenGL-Refpages/gl4/html/glTexImage2D.xhtml

https://www.khronos.org/registry/OpenGL-Refpages/gl4/html/glTexSubImage2D.xhtml