Pass texture with float values to shader

Question

I am developing an application in C++ in QT QML 3D. I need to pass tens of float values (or vectors of float numbers) to fragment shader. These values are positions and colors of lights, so I need values more then range from 0.0 to 1.0. However, there is no support to pass an array of floats or integers to a shader in QML. My idea is to save floats to a texture, pass the texture to a shader and get the values from that.

I tried something like this:

float array [4] = {100.5, 60.05, 63.013, 0.0};
uchar data [sizeof array * sizeof(float)];
memcpy(data, &array, sizeof array * sizeof(float));
QImage img(data, 4, 1, QImage::Format_ARGB32);

and pass this QImage to a fragment shader as sampler2D. But is there there a method like memcpy to get values from texture in GLSL? texelFetch method only returns me vec4 with float numbers range 0.0 to 1.0. So how can I get the original value from the texture in a shader? I prefer to use sampler2D type, however, is there any other type supporting direct memory access in GLSL?

Answer 1

This will create a float texture from a float array.

glTexImage2D(GL_TEXTURE_2D, 0, GL_R32F, width, height, 0, GL_RED, GL_FLOAT, &array);

You can access float values from your fragment shader like this:

uniform sampler2D float_texture;

...

float float_texel = float( texture2D(float_texture, tex_coords.xy) );

"OpenGL float texture" is a well documented subject. What exactly is a floating point texture?

Answer 2

The texture accessing functions do directly read from the texture. The problem is that Format_ARGB32 is not 32-bits per channel; it's 32-bits per pixel. From this documentation page, it seems clear that QImage cannot create floating-point images; it only deals in normalized formats. QImage itself seems to be a class intended for dealing with GUI matters, not for loading OpenGL images.

You'll have to ditch Qt image shortcuts and actually use OpenGL directly.