Masking a texture

[JeffM2501]

Builds a mask render texture by drawing a shape to it.
Then renders something to a second render texture before changing the blend mode to alpha only and drawing the mask.
The blend mode will cause ONLY the alpha values from the mask be written to the destination texture, thus setting the mask alpha in the render texture used to draw.

#include "rlgl.h"

#define GL_SRC_ALPHA                      0x0302
#define GL_MIN 0x8007

{
    Tx = LoadTexture("parrots.png");
    SetTextureFilter(Tx, TEXTURE_FILTER_TRILINEAR);
    Camera.zoom = 1;

    // build the mask
    RenderTexture mask = LoadRenderTexture(512, 512);
    BeginTextureMode(mask);
    ClearBackground(BLANK);
    DrawCircle(256, 256, 250, WHITE);
    EndTextureMode();

    // build the image
    MaskedImage = LoadRenderTexture(512, 512);
    BeginTextureMode(MaskedImage);
    ClearBackground(RED);
 
    DrawTexture(Tx, 0, 0, WHITE);
    DrawText("I AM TEXT", 0, 128, 40, GREEN);

    // force the blend mode to only set the alpha of the destination
    rlSetBlendFactors(GL_SRC_ALPHA, GL_SRC_ALPHA, GL_MIN);
    rlSetBlendMode(BLEND_CUSTOM);

    // draw the mask
    DrawTexture(mask.texture, 0, 0, WHITE);

    // put the blend mode back
    rlSetBlendMode(BLEND_ALPHA);
    EndTextureMode();

    // don't need the mask anymore
    UnloadRenderTexture(mask);
}


void SpriteView::OnShow(const Rectangle& contentArea)
{
    ClearBackground(Colors::DarkGray);
    BeginMode2D(Camera);
    DrawUtils::DrawGrid2D(Vector2Zero(), (int)RectTools::MaxSize(contentArea)/2, 100, Colors::Gray, Colors::DarkBlue);

    DrawTextureRec(MaskedImage.texture, Rectangle{ 0,0, (float)MaskedImage.texture.width, (float)-MaskedImage.texture.height }, Vector2Zero(), WHITE);

    EndMode2D();

    DrawText(TextFormat("Zoom:%.1f%%", Camera.zoom * 100), 0, (int)contentArea.height - 20, 20, Colors::White);
}