How do I display a grayscale image with opengl texture

2
votes

I am trying to create a texture to display. I have wxh array in which each pixel is 1 byte. I have looked at Can I use a grayscale image with the OpenGL glTexImage2D function? but I am not sure as to how to currently implement it. It looks like the GL_LUMINANCE is deprecated and I need to process the single channel independently . I am not sure how I should try this

glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, image_width, image_height, 0, GL_RGBA, GL_UNSIGNED_BYTE, image_data);

I tried changing GL_RGBA to other formats like GL_R https://www.khronos.org/registry/OpenGL-Refpages/gl4/html/glTexImage2D.xhtml. I still cannot get the image to display. Does anyone have any suggestions?

1
c++opengl

1 Answers

2
votes

If you you have a source texture with 1 color channel, then you can use the format GL_RED and the base internal format GL_RED:

glTexImage2D(GL_TEXTURE_2D, 0, GL_RED, image_width, image_height,
             0, GL_RED, GL_UNSIGNED_BYTE, image_data);

Set the texture parameters GL_TEXTURE_SWIZZLE_G and GL_TEXTURE_SWIZZLE_B (see glTexParameteri) to read the green and blue color from the red color channel, too:

glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_SWIZZLE_G, GL_RED);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_SWIZZLE_B, GL_RED);

Note, possibly GL_UNPACK_ALIGNMENT has to be set to 1, when the image is loaded to a texture object:

glPixelStorei(GL_UNPACK_ALIGNMENT, 1);
glTexImage2D(GL_TEXTURE_2D, 0, GL_RED, ...);

By default the parameter is 4. This means that each line of the image is assumed to be aligned to a size which is a multiple of 4. If the image data is tightly packed then the alignment has to be changed.

If you use shader program, then the same can be achieved by Swizzling. e.g.:

vec3 color = texture(u_texture, uv).rrr;