带着色器的SDL_Renderer和SDL_Texture？

Question

I'm working on a game with SDL2 in C++, and the current code uses SDL_Texture with SDL_Renderer to handle sprites. The problem is that the HUD is supposed to have a circular health bar, and the current thought on how to do that was to use a grey scale type image and using a shader with a float or byte passed to it for rendering. Is there any way to keep using SDL_Renderer while doing this?

Edit: Will SDL_gpu allow me to combine OpenGL and SDL_Texture-esque rendering that will be able to reuse a lot of the old code?

Answer 1

I made a switch over to SDL-GPU for my graphics, and it allows for GLSL shaders to be used on GPU_Image, which is the equivalent of SDL_Texture. The basic shader format is that below, with the image uniforms being set with GPU_ShaderBlock:

base.vert

#version 150

attribute vec3 gpu_Vertex;
attribute vec2 gpu_TexCoord;
attribute vec4 gpu_Color;
uniform mat4 gpu_ModelViewProjectionMatrix;

varying vec4 color;
varying vec2 texCoord;

void main (void) {

    color = gpu_Color;
    texCoord = vec2 (gpu_TexCoord);

    gl_Position = gpu_ModelViewProjectionMatrix * vec4 (gpu_Vertex, 1.0);

}

base.frag

#version 150

varying vec4 color;
varying vec2 texCoord;

uniform sampler2D tex;

void main (void) {

    gl_FragColor = texture (tex, texCoord);

}

This allows you to use GLSL shaders to manipulate your textures if you want. An easy example is the circular health bar as asked. The shader example I'll use gets the red value, and if it is lower than the health float between 0.0f and 1.0f, it replaces it with a semi-transparent red, and if it is higher it makes it transparent. This lets you use an image like the one below:

This is run through the fragment shader (same vertex shader as above):

health.frag

#version 150

varying vec4 color;
varying vec2 texCoord;
varying float healthFloat;

uniform sampler2D tex;
uniform float health;

void main (void) {

    vec4 texColor = texture (tex, texCoord);

    if (texColor.x <= health) {

        texColor = vec4 (1.0, 0.0, 0.0, 0.8);

    } else {

        texColor = vec4 (0.0, 0.0, 0.0, 0.0);

    }

    gl_FragColor = texColor;

}

Which gives us a result that looks like this: